First, carriers pretty much locked many people into 2 to 3 years contract. They won't upgrade until it is necessary and they most likely to wait 2-3 more generations before upgrading. I still have iPhone 4 and I skipped 4S and 5. There isn't point of it anymore. The market is stalled and no one has the money to change phone every year nor everyone wants to stack the contract up to X amount of years.
World economies aren't doing good at the moment, many people are trying to save a bit here and there. Smartphone plans aren't cheap at all, $50 minimum on average how much it costs to maintain your smartphone service.
Secondly, a few of us including myself are willing to try something different on the next round of phone shopping. Most likely I will switch to Android (Nexus 4 or next generation one) when my contract finally ends.