Ignore the knee-jerk woman-hating fantasy answer above me lol, the real reason is the mother had been more successful than the father for awhile, he wasn't making enough money to carry the mortgage on the nicer home, and was probably mostly living off his payout on the nicer property. She was the one carrying him financially for quite awhile in their marriage, his success as an author was very far behind him. Listen to the commentary track for some more background.
In most states the man is forced to continue to support his wife as well as give her most of his assets, the only way to keep his house house is to buy her and her boyfriends another one. The only thing unrealistic in the movie is that he was allowed to see his kids