Because there is literally NOTHING great about the character.
His introduction is him literally being NAMED by a white man
He loses every single fight he's in. Rey beat his ass. BB-8 beat his ass. Chewbacca beat his ass. The big ass alien beat his ass. The Storm trooper beat his ass. Kylo Ren beat his ass.
He runs away like a coward. Only to be brought back into the fold to save Rey, which he can't even do properly because while he's hamming it up with the silver storm trooper lady, Rey is saving HERSELF.
He's an idiot who thinks that you can just "use the force" to save people. Han Solo looked at him like he was a buffoon.
He's a goddamn JANITOR.
So we have a cowardly janitor who can't fight, is of substandard intelligence, and loses at every turn. His final scene is him laying in defeat while Rey goes off to train as a Jedi and Poe helps defeat the order...
What the hell is GREAT about that character breh?
Yeah, I didn't like the naming part either, but again, this doesn't have to be all or nothing, and you're taking a lot of things out of context.
For instance, I don't get the disdain for Finn's occupation when the most popular character in the series (Anakin/Darth Vader) was born into slavery. The second most popular character in the series (Luke) was a farm hand, and the third most popular character in the series (Han Solo) was a human smuggler. I mean in terms of noble professions, Finn's at the top of the heap within the context of this series.
In terms of ass beating, Luke got his ass handed to him regularly until Yoda trained him up
In terms of running away like a coward, how many times did Han Solo say 'fukk y'all' and look out for self? That was like his M.O. until LATER in the series. So why does Han Solo get a pass, but Finn is held to a different standard for his first film?
I'm not saying there aren't problems with his characters, but if you look at the way Finn's character is crafted next to the early introduction of the greatest characters in the series, he's really not different at all and like I said, he has more movies to develop, which is usually what SW does for characters