One reason is that many people don't see much of an advantage in a newer phone compared to an older one. My wife bought an iPhone 6s instead of iPhone 7 because it was £100 cheaper. She would have bought an iPhone 6 for even less money if O2 hadn't stopped selling them just a week earlier. Everything is nice and large in zoom mode (that's why she was getting a 6 and not a 5), everything runs without any delays. There is no advantage in an iPhone 7 that would count for her.