Often because (for many reasons) non-working wives feel they're not contributing as much. As the piece says, while they really are supporting their husbands' careers, too often it feels like they're the ones bring supported.

Also - I don't think husbands know how truly exhausting it is to be on call with the kids all the time. When they come in from a long day at the office, they feel entitled to some down time and because they're bringing home the bacon, it's hard to argue.