I'd have to assume them taking three years to make an 8 episode season, with the fancy expensive animation, with the ratings continually declining with every season, have to be a factor. Even if the Adult Swim guys love the show that's a hard thing to justify.
Like yeah they brought 12 oz. mouse back as a joke but that costs 37 cents to make.
@Ubiq:
It figures it'd be after the one season that had an ending that raised a bunch of questions about where the show was going. Even the first season that ended with Hank and Dean dying felt more like a proper ending than Season 7's finale did.
I dunno, a lot seasons have ended with at least some "what the hell comes next?" Generally more with Monarch than with Rusty, but season 5 ended with them all having to move locales and Rusty being rich, and season 6 didn't end at all, the Blue Morpho stuff took the first 3 episodes of the next season to resolve.