Interesting question, I've studied film/tv and history and there could be a host of cultural factors that could lead to it. I'm however more confident commenting on the Genre and TV side of why it happened.

If you look at the trends of black sitcoms you go from Good Times and Sanford and Sons depicting segregated lower class black families, then you move to The Jeffersons which shows a family transitioning to the Middle Class, then at the peak of popularity you have The Cosby Show and a synthesis of these themes in The Fresh Prince of Bel-Air.

It is pretty typical for a genre to evolve then hit a point and people lose interests after the tropes have been explored (Think of Westerns, very popular then declined and had a brief resurgence recently)

There is another genre reason this could have happened. Family sitcoms lost popularity in general during the 90's with the rise of Seinfeld, Fraiser, Friends and other shows, while many family sitcoms were animated (The Simpsons, Family Guy, King of the Hill.)

This probably has a lot to do with children. Children are difficult to work with and have lots of laws limiting their film time. They age and become less cute. Then they hit puberty in weird ways. Finally, they want to go off to college. However, a group of adult actors can be consistent for a long time and children on animated series never have to age and can be voiced by adults.