The anomaly with some iPhones being available and others not available might be due to stock levels. Maybe they are sold out of iPhone 8s and have no idea when more stock is coming?
I was wondering if it's something perhaps a little simpler still: reducing the number of items available to reduce complication in a difficult time for retailers
iPhone 11 - top end
iPhone 7 - bottom end
Nothing in between...you choose - top or bottom. Both work, both available.
Maybe I am just oversimplifying it, but that's my hot take of the day.