Sometimes it really IS as simple as he is just a selfish git who cannot hack the responsibility of being married with kids.
Funny how men seem to find it so easy to just walk away from their responsibilities/ wife/children. And it's often another woman, (as a pp said, men are too cowardly to leave with nowhere else to go/no-one else to turn to.)
And he will usually leave for a pathetic reason, like he is 'stressed' (like the wife ISN'T!) and the marriage was on the rocks, coz she 'pays more attention to the kids than me,' and 'she isn't the woman I married.... etc etc.' God forbid a woman takes off and leaves the kids though. She would be classed as evil itself.
I have seen many a man just pack a couple of suitcases and piss off with his OW.
Funny how they never seem to even attempt to take the kids though. Probably get in the way of their fun and games and shagging and entertaining other women! 
I know a few couples right now where the man has buggared off with another woman, and left the wife and kids, and then a year or so down the line things weren't so great with the OW, and he sees his ex with another man (who she met after he left her,) and he tries to get custody of the kids. In most cases though, the kids think he is a cunt and don't want to be with him.
I also know a few men who have pissed off and had hardly ANYthing to do with the kids after.
Utter bastards! As I said, it proper fucks me off that men find it so easy to walk away from their children. Good job women don't find it so easy. And I hope no-one comes up with 'I know this woman and that woman who left her kids' garble, coz most women don't! Men do though!