Family Guy was this horrible bizzaro version of The Simpsons, and Fox put all their support behind it. Whereas Futurama was this really funny, inspired show and they flush it down the toilet. The worst thing you can say at The Simpsons about a joke is that it is Family Guy. It means the joke just pointed out the obvious, and we can do better than just the obvious.
Personally I would love to see Simpsons vs. Family Guy escalate into an inter-show feud; those are great, especially when one show is demonstrably superior to the other (and even at this point, The Simpsons still beats Family Guy, particularly since The Simpsons, unlike FG, occasionally manages to attract a viewer over the age of 23).
My favorite story of this nature is of two shows that were being produced by MTM in the late '70s, both created by fairly young writers: Hugh Wilson's WKRP in Cincinnati, and Gary David Goldberg's The Last Resort, about college students who take summer jobs as waiters at a resort hotel. At some point Wilson decided that MTM was giving Last Resort preferential treatment, and wrote an angry letter to Grant Tinker (president of MTM) accusing him of as much. Tinker's even angrier reply is printed in his autobiography. Anyway, the writing staffs of the two shows worked in the same building, and they would do things like (in the case of the WKRP staff) sneak into the washroom and paint "Why hasn't America embraced a sitcom about surly waiters?" on the wall.
So from all Family Guy haters, or fans of feuding TV writers: here's hoping for much lavatory defacement to come.
Incidentally, Selman's statement that Fox "put all its support" behind Family Guy is pretty much accurate; in early 1999 Futurama was expected to be the hot new animated show, but Fox instead gave a bigger buildup to the less-anticipated Family Guy, giving it the post-Super-Bowl launch and the post-Simpsons time slot. At the time, it looked like a bad decision, since once they moved Family Guy to its own time slot, it tanked (it's not just that it didn't win, it's that it got lower ratings than the show that had been in its slot before, a clip show called "Fox Files"). This led to the myth that Fox had killed FG by "moving it around."
Since Family Guy has become such a moneymaker, I suppose that in retrospect it turns out that they bet on the right horse. Like I said, it's not the first bad show to become a hit, nor will it be the last.