Because Thanos is simply stronger than the Hulk.

That statement is not made without controversy, but according to the film's writers in a MovieWeb article he didn't even need the stones,

Avengers: Infinity War has brought up more than a few debates, but screenwriters Christopher Markus and Stephen McFeely have now revealed that Thanos would've beat the Hulk even if he didn't have the Infinity Gauntlet with the Power Stone, fueling more debate. The opening scene takes place on the Asgardian refugee ship after Thanos and his Black Order have taken it over to locate the Tesseract. In a last ditch effort to defeat Thanos, Loki sends the Hulk into a rage to fight. However, the fight doesn't go in the Hulk's favor.

Stephen McFeely believes that the Mad Titan would've destroyed the Hulk with his own bare hands in Infinity War. This would explain why Ebony Maw allows the fight to continue and makes the scene look like Thanos was just having some fun, like a cat playing with a mouse. Looking back at the scene, Maw and Thanos are pretty calm, even when it appears that the Hulk has the upper hand in the battle.

McFeely had this to say when asked if Thanos could still beat Hulk without help. I think he could. I may be speaking out of turn, but I think Thanos could kick the Hulk's ass without the (Power Stone).

This was to establish that Thanos is not to be trifled with,

However, the opening Infinity War scene wasn't just to show that Thanos could beat the Hulk. Marvel Studios boss Kevin Feige revealed early on that the first 5 minutes of Infinity War would show off the immense power of the Mad Titan and he was not kidding around. McFeely explains.

From an in-universe perspective we also have Korath the Pursuer state in Guardians of the Galaxy that

Thanos is the most powerful being in the universe.

This is when Ronan is about to wield the power stone and Korath is trying to stop him.

You can also find the source interview for the above linked article here.

They appear very evenly matched in terms of raw strength, unfortunately for Hulk, strength wasn't the deciding factor in this fight.

Hulk lays out Thanos initially, and Thanos isn't able to push him off - until he's got something to brace against. In a contest of strength between two opponents of approximately equal strength, the winner is going to be the one with greater momentum or something to brace against.

Thanos uses this momentary advantage to get to his feet and set his stance, after which he controls the fight. The Hulk becomes a glorified punching bag because he fights without technique, and against an opponent like Thanos, that's not going to work.

Hulk consistently wins because he's so ridiculously stronger than most of his opponents that his capacity to absorb damage means he can basically ignore technique with the same freedom that he ignores damage.

Against a disciplined technical fighter in his own weight class? He never stood a chance.

TLDR: We don't know and can only speculate. The movie uses this to get our curiosity and to establish Thanos as a strong villain.

We never see the Hulk build up anger. For all we know he could have just woken up in his bed, realising the ship is under attack, smashing a few of Thanos underlings and then go right at him, just because he was awoken in the middle of a good dream. At the least, we know he typically gets angrier if he's in life-threatening situations himself or if the love of his life is in danger. In the MCU, Black Widow gets closest to that and she's not around, nor do we have an indication that he was in serious danger before being beaten by Thanos.

So in-universe, it is likely he wasn't at his full potential and not very focused. And Thanos is likely at least about the same strength level, with better technique (less emotional fighting style) in that fight. Hulk was probably taken by surprise in how strong his opponent was - and we've seen in Age of Ultron, that surprising him when he's not looking/focused/at full strength is a sneaky way to get him out of the picture for a while.

Hulk wasn't ultimately defeated in the sense of about to die, yet a space ship is also a terrible place for the MCU Hulk to fight in. So far, we have no indication to think that he would survive in space. Even if that were the case, breaking the ship apart would kill the remaining Asgardians. And this would likely happen if he were to fully "hulk out" and have an extensive fight against someone at least equally strong. So, in his "mentally improved" state where he can talk and recognizes "friends" properly, he might also have held back on purpose. In any case it was probably a sane decision to just send him away and not drag out the lost fight.

Out of universe, the movie explicitly doesn't focus on him or his mental state. What mood he is in, what fights he had earlier whether he just transformed, whether he already is in disagreement with Banner or not. It doesn't make a big fuss about the fight and none about any "preparation" on Hulk's side. This way it builds the question of how he lost and why he hides inside Banner up as a mystery box for the next movie to open and investigate, while for the time being using him and Thor to establish how powerful Thanos is. Up to this point, those two were the top-contenders for "strongest avenger", so it sends a powerful message if Thanos outright beats both(!) - individually.

Personal side note: I sure hope Hulk can pick up that Gimli-Legolas relationship again, solve his mental issues and "level up" as well, after Thor stepped up his game in Ragnarok and IW.

First of all this is not the Hulk of the Comics and so one can't be compared to another.

Within the MCU we have already seen Hulk beaten or matched in combat several times.

Thor is his match in the Avengers and in Ragnarok again proves to be able to match a Hulk who has been fighting for years finally beating him to a draw.

Tony Stark in the Hulk Buster, yes it took dropping a building on his head but he was able to defeat Hulk in a fight.

So within the MCU we have already seen Hulk is not as infinitely strong as his comic book alter ego. Now add to this the fact that Thanos has been defined as the most powerful being in the Universe, he has an infinity stone already, has been shown to beat Thor (who could hold his own against Hulk) and is therefore easily a match for Hulk. Remember Hulks power comes from Anger, now if Hulk is being beaten by Thanos fear and doubt starts to creep in. This then leads to these emotions overcoming his anger meaning he would actually weaken meaning Thanos can cause more damage leading to him weakening more.

This fear and shock at being so easily beaten would explain why Hulk does not want to come out to face Thanos again later in the movie.

This was discussed by the film's directors, the Russo Brothers in a recent interview. In short, Thanos is as strong as the Hulk and more than a match for him in terms of fighting technique. The only thing Hulk has going for him is raw aggression and that got him a slight advantage at the start of the fight along with the element of surprise.

Note also that Thanos wasn't using the Power Stone to augment his own natural abilities. Had he done so, presumably he'd have been able to punch a hole straight through the Hulk and out the other side.

I would say that he’s just that powerful. You didn’t see him actively
use the power stone in that fight. I think ultimately, the way we
looked at that fight and the way talked about it with our stunt team
when we were executing it was Hulk is obviously very powerful, but
he’s a little mindless in his fighting style. It’s aggressive; it’s
pummeling. Thanos is the Ghengis Kahn of the universe; he’s a very
skilled fighter, equally as strong. So when you put those two up
against each other, the more skilled fighter is going to win
ultimately. Which is why Hulk has a moment where he overpowers Thanos,
but ultimately Thanos is smart.”

_{^{The Russo Brothers Share How Thanos Beat Hulk In ‘Avengers: Infinity War’}}

He did use the Power Stone. According to the comic book, Thanos is able to wield the power of the stones simply by having them in his possession (he doesn't need to keep them inside the gauntlet). With the Power Stone, Thanos' physical strength is basically unmatched. At some point in the storyline, Thanos admits that he needed the Power Stone to overcome creatures such as the Hulk, implying that at his regular strength level the Hulk would be a match for him.

Still, based on feats, I don't believe the Hulk would be a match for Thanos even without the Power Stone. I've read all issues of the Incredible Hulk comics and there have been dozens of creatures who have defeated him in combat. People want to think that Hulk is invincible and the physically strongest creature in MU but it's simply not true. His strength is proportional to his anger, but since his anger has limits, so does his strength.

On the other hand, very few creatures have bested Thanos in combat. The only person who has achieved this single-handedly in a straight-up fight happens to be a character from the MCU who has conveniently gone missing during the Infinity Wars (of course I'm talking about Odin).

The creature who has overpowered the Hulk more easily than any other (and actually killed him in the split of a second) is a character who has made cameo appearances in the MCU (talking about Cosmo the Spacedog).
PS: Alright, I know that was Cancerverse Hulk - but it's an overpowered clone of the real Hulk...

Actually, the real reason Thanos beat Hulk is a really sad and surprising. When Banner was trapped in his Hulk form. Hulk finally started to age really slowly. You have to remember, time works differently in Sakaar (Thor Ragnarok movie). So Hulk was gone from earth for four years. But to him, It was a lot longer, long enough for him to become intelligent.

So basically, Hulk is getting old. And when you get old, you aren't able to do the same things that you could do when you were younger.