It's an interstate show that takes place in the middle of the transition to more modern situations.
Lois specifically works at a terrible job part-time. I remember her going on a rant mentioning that she works 39 hours a week, which is them saying the store works the staff as hard as possible while denying them full-time benefits. But they also manage to pay for boarding school for Francis for a few years of essentially 1.5 incomes.
But even after Francis has moved out on his own, Malcolm and and Reese have to eventually get jobs to help support the family.