The Empire left Hammerfell out to dry, and the Stormcloaks want to ally with them to keep fighting the nazi elves. Like all the factions are vile monarchist shits, but a national liberation movement that's explicitly fighting for solidarity with other nations in their struggle against omnicidal elven fascist imperialism seems like the lesser evil compared to an imperial machine that is itself ruled by compradors collaborating with the omnicidal elves.