For some, people either keep or replace their phones based on the features of a new phone. I think it's be particularly "difficult" this year because we have had waterproof phones (Sony), "premium build" phones (HTC), feature-packed phones (G2/S4), cheap phones (Moto G/Nexus 5), and some other phones that have a few new features but also stick to a solid, tried and true formula (Maxx and Moto X).
I've gotten a few devices this year just to test out these different features, but I think phones are built so well now that it's not unreasonable to stick with one, particularly these Motorola devices seeing as how they are being supported so well.
Probably really comes down to priorities and money though, and many don't want to dish out the money or spend the time to find a cheaper (used) device when they are content with their current one.