From what I understand, about ten percent of the American populace actually supported revolution against England. The world knew it was a foolish move to challenge the mightiest empire on earth, especially with the untrained and undisciplined army available. It was simply idiotic.
But it worked.
A large part of that is simple geography. America had an entire ocean protecting it from England, and it really had no nearby European countries to meddle in its affairs. They were able to fight on their home turf, with all of their resources immediately at their disposal (relatively speaking), and wore the British down by not engaging them in direct combat, but rather through the Fabian Strategy and fighting a war of attrition to dwindle British resources to the point that it just became too costly to be worth the effort.
Then, as Celan pointed out, the French loved screwing with their British rivals, and began supporting the Americans following their victory at the Battle of Saratoga. The British use of Hessian mercenaries also turned many Tories against the crown, and some of the Hessians even ended up deserting the British to either just settle in America, or join the Revolution instead.
Ulfric doesn't have geography on his side, he's not fighting a war of attrition, and he doesn't have any allies who are helping him out to screw over the Empire. The Thalmor might be interfering with the Civil War to prolong it, but that's merely keeping the status quo, and not helping Ulfric find victory either. He can't even use the Imperial Legion for extensive propaganda either, considering the Legion's recruiting most of its soldiers locally, so it isn't so much foreigners coming in and smashing up Skyrim as it is Nords rejecting Ulfric's claim to the the throne. His biggest asset is weather, and even that won't do him too much good, considering the majority of the enemy forces are his own countrymen, who are well used to Skyrim's bitter cold same as his are.