Hard to say. I suspect that some players (myself included) played D3 as their first real venture in the series, and simply realized that the game doesn't have compelling replay value.
I think that the changes made to the game recently have been positive.. but until an expansion or new content is released, I just see no reason to go back. Regardless of prizes, you're seeing the exact same story play out, with the exact same encounters.
Some sort of map / scenario building tool for the community to make their own content would have done wonders, but sort of flies in the face of how this game works.
Solid game despite everything, and I certainly got my money's worth, but this just isn't a genre that would keep me playing for months / years.