Family Guy was never originally good with the ratings. Fox's schedule shuffling was an attempt to keep it alive (not purposely kill it like you think) by finding an audience that could compete share-wise with their other programming. Re-airing on Adult Swim saved it because it allowed it to gradually build a fan base at a pace that bigger networks (which compete with other bigger networks) can't afford. I too, originally, didn't like it but now I'm a huge fan and I'm glad it came back.
As far as why Cleveland, why not? Who are you to say what the writers feel they have more potential with? Are you a successful animation creator?
This reminds me of the time I fucked your fat mother (insert family guyish non-sequitur here).