jobs paid nothing and 'no man who was a man' wanted to stay home to raise kids and clean house. It was also social death to be a divorcee. So Mom stayed home, due to combined economic and social pressures.
It's MEN who mostly 'assume this parent should always be Mom.' It's also men who ask "Why not dad?" You'd have to ask men why. I'm sure there are several reasons -- housework/parenting is still thankless work, even if you're a man; if you stay home you are disadvantaged economically and power-wise in the couple relationship; men, like women, feel fulfilled by work that uses their talents; most women still make less $$ than men, so they end up as the housekeepers by default; and recent studies show that men who are unemployed do LESS work at home than employed dads, so Mom herself may have serious reservations, based on experience, about letting Dad be the stay-at-home parent.
The chore wars are still being fought, and I think that women are very leery of househusbands because so many working couples already have problems dividing up (or even agreeing on what, exactly, should be done) the daily chores equitably. Women don't see that problem going away - they see it becoming magnified, with them having to do the second shift at home when Dad still doesn't 'see' the dirt on the floor, the laundry pile, the dirty bathroom towels that don't get changed out, etc.
Now, having said that, I know two families who do this and it works out fine. But both dads REALLY know what needs to be done daily, weekly, etc. Maybe if enough men become househusbands, the job will get some respect. God knows it doesn't get much when it is mainly women who do it.