As far as I know there are two reason for short seasons:
A third reason: Incompetent, moronic network executives who cancel the best TV series ever run halfway through its first season after changing the sequence of the episodes, changing the time slot, and failing to promote the program.
I was astonished that Joss Whedon gave Fox another chance after that debacle.
Well, I think he was referring to the short seasons of UK TV, which is what we've been arguing over, rather than short
ened seasons, which is what we get from good shows in the US

As for Joss, I believe the latest news is that he might do a show for FX. Maybe Fox doesn't have him under contract anymore...