User blog comment:ShawnCognitionCP/The Empire vs Stormcloaks, the most likely answer/@comment-9062114-20140611211441/@comment-12599067-20140613152742

Pardon me, but since when does one Jarl's word compare to the will of an Empire? A king cannot please all his subjects. He decided to please both the Empire and most of the Jarls by siding with the Empire, rather than secede and face the possibility of a Thalmor or Imperial invasion. It's sort of a "needs of the many outweigh the needs of the few" kind of thing.

Er, Hammerfell seceded from the Empire. It was not rejected. Also, not all of Elsweyr has submit to the Aldmeri Dominion's rule. I believe when Elsweyr split, one of the halves (I believe Anequia) sided with the Empire.

Ulfric will receive no support from Morrowind. The Dunmer of Windhelm are oppressed by the people of Windhelm, who (in a way) reflect their Jarl's strength as a leader. Not to mention the fact that the Dunmer and Nords have never been on good terms. Hammerfell and High Rock aren't necessarily fast friends with Skyrim either. May I remind you that Hammerfell is a collection of kingdoms ruled by two factions, so assuming they're not in a civil war, it'll take some effort to wind them over. High Rock is a number of kingdoms as well, so unless Ulfric can defeat the Empire without allies, thus proving himself, I doubt they'll all side with him.