And by that I assume you mean they're a bunch of dragon-slaying, devil elf slaughtering conquerors who took a break from being badasses up north just long enough to save the Cyrods from slavery and help them create another Empire?
But seriously, the Nords have Nordified Cyrodiil way more than vice versa. That's why the gods mostly came from them.
I honestly don't buy that this happened. The Imperials were never able to impose greatly on Nordic traditions before the time jump. As of Oblivion, Cyrodiil has tried and failed to bring more of their own influences north, and the Nords have stubbornly resisted. Not long after that, the Empire totally collapsed and lost its control over everyone. The Medes didn't start to pick up the pieces until decades later, and even then they never had he influence that the Septims did. What kind of sense does it make for the Nords -who resisted Imperial influence under the Septims, who thy respected- to suddenly worship that culture under the Medes, who they have no real reason to care about?
I think it's just a matter of Bethesda sucking at depicting the cultures the way they write them. This would seem like a cop-out answer in defense of the Nords if not for the fact that they have a history of doing so, and even with their last TES game in Oblivion, the Cyrods had almost no discernible identities or cultures, despite there being plenty of contemporary lore that said otherwise.
I have to hope that even Bethesda aren't so scared of consequences that they would see why it would be poor writing to destroy yet another game region after having us save it. It worked well with Morrowind and even Oblivion because the events of both those games heavily foreshadowed uncertainty and impending doom. Skyrim, no matter who you side with in the war, has a much more optimistic ending. Doing it again would just be lazy.
"What's the fine in Cyrodiil for necrophilia? ... just asking."
"Is it the first offense?"
"Let's assume no."
"Then its at least five hundred gold."
"That's nothing compared to Morrowind! Thanks!"
Bethesda likes their creepy necro jokes.