Their priorities shift. Many enjoy cooking for their mates and therefore prepare and eat more than they did before marrying. Many take on new responsibilities — earning income or raising children — that lead to rushed eating or loss of exercise time.
The rewards for weight maintenance — attention, compliments, gifts — often peter out or stop when they marry.
And they get older, which makes weight management harder.
Husbands who care about their wives’ weight can make exercise, a good diet, and time for them a priority for both of them. (The folks at the end of wishing our days away are great role models for this.) They can also keep the rewards coming, maybe even increase them in recognition of the extra effort required.