Because people are lazy.
Is it that bad? I don't have a Vita so I can't say. From what I heard it works pretty good, although I don't know about things like latency etc. Whatever, just develop it for both systems and make it Cross-Buy, then, so you can play the same game at home and at work.
Look, I'm not saying that handheld MH is a bad thing. I just think that a console MH could actually use the extra power to do some amazing things with it and that that would make one awesome game if done right.
Remote Play works better than fine, the only problem i've encountered so far is if the game has a quite small UI, that translates to a .. quite problematic vita experience. It wasn't that easy to read the numbers in your toolbar in Don't Starve fx.