Fox never seemed to understand it, but they wanted to keep Matt Groening happy so they kept it on. But then they deliberately put it in a time slot where it often got pre-empted by NFL games running late. Then they kept trying to justify getting rid of it by saying it got low ratings; when the reason it got low ratings was half the time it wasn't on so it was hard for a large audience to catch on.
And I have to admit I rarely watch The Simpsons anymore myself. It wouldn't bother me much if they did cancel it; I think that it's had a great run but it's getting tired. Last summer when the movie came out The Onion's "AV Club" even had a point/counterpoint article debating if the show should be cancelled. That was where I first learned there are a large number of fans who won't even watch episodes from after a certain point. (I.e., only the first seven seasons, that kind of thing.)