For me, it's a combination of fatigue and not really liking the newer material.
The first playtest had its hooks in me with old-school complexity (different complexity levels for different characters, varying from really simple to still pretty simple) and the advantage/disadvantage mechanic. Every playtest since then, I look over and feel like they're not really going to be much simpler than 4e or Pathfinder... at which point, I see no reason to switch.
I'm mostly just sticking around to see if they surprise me. I've been impressed with Mearls's designs before, so there's a good chance he'll wow me with something.
But, really, I don't see how I'll end up playing 5 from the playtests. Which I'm more than fine with, I just hope that they provide some people with the game they want, even if none of those people are me.
Cheers!
Kinak