New iPhone models are not selling out like they use to. Analysts have determined that with each new iPhone, customers are getting less excited. When the iPhone XS and XS Max went on sale the lines outside of the store were smaller than usual. When the iPhone XR went on sale at 3am just about all the phones were still in stock when the sun came out which was a change from iPhones past.
Could be that the smartphone market has expanded so people have more options now.