Board Thread:General Discussion/@comment-97.80.69.248-20140817000615/@comment-7203512-20140818073442

"One of the most politically unstable but militaristically among the most powerful alliances of Tamrielic history, the three cultures that comprise the Ebonheart Pact shared a great distrust for each other, only holding allegiance to one another because of a common rivalry with Akaviri invaders."

- Ebonheart Pact

That explains everything, I guess.

The Elder Scrolls V: Skyrim is set in the Fourth Era, a time where no such alliances were necessary. I'm pretty sure the Nords would've acted the same as in Skyrim if they wouldn't have stood before a common threat.