Lots of speculation - but the cause could be almost anything. While we can postulate scenarios involving design priorities, licensing issues, and other things, it could simply be something like a contract renewal and Monte not being satisfied with its terms.
There are a thousand possible reasons, and Monte doesn't sound likely to share them. Obviously, that means we'll speculate and conjure different scenarios, and one if them might gain traction and become "accepted" but only Monte and WotC know what the issue really was.
True. It seems that if we did not know Monte was working on D&D 5e at WotC and we met him at a cocktail party and he said "Yeah, I just left my job, I can't really talk about it" that our first conclusion is because of design issues?
Apparently not as many of us have had crappy bosses, poor pay, oppresive work regulations, etc.
it can easily have been a matter of Monte was promised to be made a fulltimer and not a contractor, they didn't, he had a better offer in the works, so he said "See ya!"
Because that never happens in the real world...
I haven't been following the 5e threads. I have no clue if if was sounding like a good game. In reality, 4e sounded like a lot of good ideas before it hit the market, and I heard a lot things I didn't like. So ultimately, 5e doesn't matter until it's released, and I find out from other people if it sucks or not.
It's too bad Monte had to leave, but if he wasn't happy, how much of that unhappiness would have made it into the product anyway?