What the thread title asks. Why? Did he manage to piss off Shor somehow? Is this event responisble for the decline in magic use among Nords?

UESP is useless here; it simply says that he became Julianos after falling out of favor with the rest of the pantheon. Based on Skyrim's environment, though, I was under the impression that the Nords didn't use magic partly because the Mages Guild dissolved and the College of Winterhold "proved" that magic was dangerous via apparently destroying the city around them. It might be that the opinion of magic Nords hold in Skyrim is older than it at first appears, of course, and there's nothing to suggest that Jhunal was more happily received in the relatively recent past than he was in the present of Skyrim, so clearly I'm not getting the full picture here.

Hmmm....

Some started a thread a while back about the Nords getting dumber. Point being that there old ruins seem much more technically advanced than their newer buildings.

If we accept that gods are the concepts they stand for, the Jhunal going out of favor is like "knowledge" or "learning" going out of favor.

Between their constant wars with magically adept races, the Dragon Cult usurping all the other forms of worship, and the eventual war with said cult, the Nords lack of interest in magic isn't all that surprising.

I think its also worth noting that Jhunal eventually became Julianos somewhere down the line, most likely with the advent of Alessia's pantheon. He isn't found in any of the Aldmeri pantheons, and may be not to dissimilar to Akatosh in that regard.