All the hardware has reached the point of letting people use Facebook, Email, Take HD photos, and use GPS navigation. A huge number of consumers don't have a reason to upgrade.
Personally, I have a S3 and I will stick with this thing for a while well beyond my contract ending. If I do switch, it will only be because Sprint finally released a decent Windows Phone. I'm only willing to switch to try a completely new phone experience. It's hard to get excited about getting a new phone but staying with the same OS you're on now.