I don't know why people would rather watch a Quadmire spin-off, it's not like he does anything great, sure he's funny as hell, but he's just there for the casual sexual joke. And I think this looks great.
People want Quagmire because he is funny despite being a one joke character, as opposed to Cleveland who isn't funny at all despite having more marginally more material.
The reason they brought family guy back was fox saw how well it was doing on CN/AS. At least thats what I heard anyway. Some of the new eps have been pretty good.