I'm pretty sure Trump just views primary debates as a chance to give the other candidates free publicity, and why should he do that? He figures he's entitled to the nomination and leading anyway.
The problem for Trump is that those debates are going to proceed anyway, and a lot of GOP voters are going to watch them. The other candidates likely will take a lot of shots at him, and he won't be there to answer. So they're going to get that free publicity one way or the other.