If both mom and dad are Hispanic or black, mom is more likely to be hired for a manual labor, low wage job. It happens even if dad has a college education. If he applies for work commensurate with his education, he competes with white and Asian candidates. (The public sector is an exception, but there are only so many positions available.) If dad tries for a low wage job, he is turned away as "over-qualified.". Mom is more likely to lack higher education, therefore ends up working several low paying part time jobs while dad stays home with the children.
Before everyone begins to down thumb me, please note that I'm speaking in generalities, not specifics.
Feminism is also to blame. Many times the woman has a college degree, starts a career, then decides in her late 20s to 30s that she wants to be a mother. She finds slim pickings among college educated men of her age, so she settles for a younger man or an immigrant. She marries , gets pregnant, and discovers that she can't or won't live on her husband's income. So it's back to work and dad becomes a house-hubby.
Whatever the situation, it is certainly not optimal for mom to go to work and dad spend his days changing diapers and managing play dates.