I think there's some good answers here.

My own take on it was that the Six Duchies were the focus of most of the books, and given the taboo nature of the wit, the characters there may just have not known about it elsewhere as it wasn't something to be studied and discussed. That said, it does seem that both the skill and the wit came from the Elderlings and are two sides of the same coin.

It's been a couple of years since I read the series again and I'm kinda hazy on some details though! I mostly just remember how satisfyingly the series concluded despite it being incredibly bittersweet (I clearly remember wondering WTF to do with myself next after finishing xD).