because after the civil war no matter what side ur on, with the replacing of the Jarls EVERYONE is on the same page and not divided. Ulfric dosent have to go thru the other countries for the fact that Nords are excellent "navy" boat fighters and are gonna sail from Skyrim to the Thalmor's island itself to fight...dotn have to go thru any countries to do that.
So the Nords will leave Skyrim undefended so that the Imperials from Cyrodil can reclaim it again. Makes no sense from a strategic point of view.