Because the Empire showed no signs of fighting them in the 30 years they waited, and all that time consisted of the Talos ban.
Perhaps is because Im not a religious man in any way, but the Nords should have know that in the end, they must fight back. And that the only way to do that is to avoid any interlan conflicts and to join forces.
I cant belive how nobody from Skyrim just go to skyrim and sayed "Well, if you are not going to fight the Thalmor we will do"
If Ulfric, instead of using his Stormcloacks chasing Imperial troopers, should have just leaved Torygg alone, selected an army of bloody and crazy Nords and just jumped into Dominion territory.... like Ysgramonr and his 500 !!!
That would have made me thing that Ulfric is a damn HERO , the best of all, the glorious warrior that go to his battle just to defend his beliefs !!!! But no. He is not that guy , sadly.