I suspect elements of the following:
Diehard fans have already plumped for the XS, Max or last year's X, thus lesser demand from the traditional early adopters.
Plenty of stock, as Apple will have had plenty of time to gear up towards this. It's been Angela Arhendts' (head of retail) mission to get rid of the queues outside stores by giving people ample opportunity to order day 1 delivery.
Mostly everybody else will just wait for their upgrade cycle to come around. Two friends of mine went and bought new iPhones 7 and 8 in late August because their contracts were up or they desperately wanted a new phone, despite me telling them to switch to SIM only plans and wait until September for the announcements. They weren't bothered and were just happy to have a newer iPhone than their outgoing models (5S and 6S).