$1:
“Modern couples are just that, both in the way they divide up the chores and in their perception of marriage,” the co-author of the study, Thomas Hansen, told the Telegraph. “In these modern couples, women also have a high level of education and a well-paid job, which makes them less dependent on their spouse financially,” and therefore less apt to be trapped in an unhappy marriage.
I think that goes a long way to explain it. I would further add that households where the women do all the work are probably not happier households but rather religious/socially conservative families where divorce is considered taboo.