I am surprised this show has lasted long. It's ratings are below average than what a primetime animated show should be getting. It gets about 2-3 million a week which would be huge for cable but for a sunday night show that is a spin off for one of the biggest cartoons around, that is poor.
I think FOX got a little cocky and assumed they had a hit on their hands as it was spin-off from Family Guy because The Cleveland show was given a second and third season before the first season had even finished airing.
Fox have gotten rid of shows that have given better ratings than this show.