In the end, it really doesn't matter why, does it? He isn't a good marriage partner, and he doesn't want to be in the marriage.
I'm not trying to make light of the pain you feel--but the bottom line is it DOESN'T MATTER WHY.
If you can get to a place where you are able to accept the situation (without understanding it or approving of it), you will start to know peace and be able to go on with the great life out there that is waiting for you.