Because he can't. You can't be asked to put in that much defensively and still be as influential as he normally is in the final 3rd. He's being played out of position and in a role that doesn't suit him. His reaction is understandable because he's never going to be able to play how Jose wants him to and be good enough at it, something I think everyone could see at the start of the season.
We had hoped however, that he would be used in rotation with Oscar and used to break down tough defences. Instead, he's just being told to put up with it and play that way or get out. If Jose knew in the summer that he wanted us to play this sort of game again, Mata should have simply be sold, not given false hope.
It's depressing as a fan, because if we're doing that to him it's obvious that we're not just going to play tight, efficient football in the toughest matches, but basically all the time. It worked the fast time when we were fast and physical and powerful but we're not any more.
Come on GC, you know that this is bollocks. With Sturridge it was a problem with attitude, with Mata the problem is that he can't exploit his biggest strength (finding space) if he's having to track back all the time. He can't just will himself 20 meters up the pitch again everytime we get the ball back.
tl;dr: use him properly or sell him. The guy doesn't deserve it.