My number one reason is simply the fact iPhone is built and sold by one company. This is a plus and a minus, but in the long run, amortize cost makes that a bigger plus than a minus. My experience so far, iOS devices can live up to at least four major OS updates. That's at least 2 years. I think my iPhone 3GS lasted about 4 years before I switched to iPhone 6 (at that time the power of 3GS was simply too slow to handle map and instant messaging).