I too prefer
American Dad to
Family Guy. Honestly, AD is probably the best animated show Fox has right now... so naturally, it's the least popular.

Audiences.
I never thought
Family Guy was all that great, but at least it could occasionally be funny. This past season, however, was painfully unfunny for the most part -- in fact, it pretty much sucked. Of course, this is the season that it gets an Emmy nomination for best comedy. Not just animated series...
comedy. There hasn't been an animated show nominated in this category since
The Flintstones, and they pick
Family Guy to be the first? God, what a joke the Emmys have become.
South Park would be much more deserving... and ten years ago,
The Simpsons would've been as well.
I've noticed that a lot of people I know still think
Family Guy is great, which boggles my mind; they can see that
The Simpsons is nowhere near as good as it used to be, so how can they be blind to FG's many, many faults? It's a mystery...