It seems to me that when the show first started out, Bart was conceived as being the central charachter of the ensemble, like Goliath on Gargoyles, or the Captain on any Star Trek Show.
Whether by accident or design the writers gradually started gravitating towards Homer and he became the main charachter, demoting Bart to suppurting cast member. Plus, even when part does got a lot of focus in an episode, nine times out of ten, it deals with his realationship with Homer
One easily sees this reflected in the merchandise; back in the early 90s Bart got his face plastered on t-shirts, posters and any thing else Fox could hock. By contrast, look at most Simpsons merchandise made in the past decade and we can see that it tends to be rather Homer-centric.
Basically Homer is the Fonzie of The Simpsons, a secondary character, who's unforseen popularity caused him to eclipse the rest of the cast.