It could be a slightly lesser effect; people were willing to buy the S models of previous iPhones, even though the changes weren't that big on the surface, because the price was lower, however when the phone is so expensive, people wait for a bigger change before buying a new phone. The iPhone S was a gigantic change relative to all other new models of iPhone; the Xs is mostly more of the same.

Or that might not be the reason, I don't know enough to analyze the situation by far. I just don't think we should rule out the extreme price increase as a potential partial explanation.