Well, because they're starting to get caught up. We are still seeing all of the store stock getting snapped up very quickly both here and the US, so there's still pent up demand (but I think the majority are those waiting on pre-orders)

So, every time a pre-order snags one that was destined for store stock, the line gets a little shorter.
I think the 2018 estimate for being able to walk into an Apple store and pick one up was over-rated....but, maybe we haven't seen the Christmas rush yet.
Edit: Before someone corrects me - please insert your favourite denomination's holiday where I so callously placed "Christmas" in that last sentence.