Too many people sell their old phones for decent money. Others (like my sister) like their two-year-old models with all their data on them and see no real reason to upgrade, so when they're offered an upgrade, they sell the new phone.
Others (like me) just don't want to put that much effort into it. It's a phone. If I can call people, check my gmail, use hangouts, use google maps, look something up, and maybe use the camera, I don't really need anything more. I can mod my Droid 4 when I need to, I think I could run over it with my car without too much wear, and it has a REAL keyboard. You can have it, when one of us dies. I have no time or desire to learn a new OS either. I use a PC; my Droid is kind of intuitive.
Plus, the little Android dude is adorable.