Because it's not open-world?
Think about it. You can still create and customise your character and make decisions which affect his/her story. So essentially, roleplay. Therefore, it is, in my books, an RPG.
RPG is about "role-playing".
If you compare ME2's role-playing aspect to New Vegas then New Vegas wins by far.
ME2 might have better graphics, story (I don't think so but I'm neutral atm), interactive dialogue and characters.
But does it really have better role-playing aspect?
In New Vegas you get four main paths.
In ME2 you get one.
In New Vegas you can be whatever you want to be (pre-courier that is).
In ME2 you get to be Shepard und only Shepard.
In New Vegas you can kill anyone you wish to so that you can be a rampaging psychopath, a silent assassin or kill someone for whatever reason you want to.
In ME2 you get to kill the people that the game "wants" or "lets" you kill.
ME2 might be an awesome game (I just think it's good, ME1 was awesome though) but it's not as good of an role-playing game as New Vegas is.
Just like how I consider FO3 an awesome game on it's own merits but an awful Fallout game.