I think she got burned out during the last campaign; I think she enjoys her current role as a spokeswoman for conservatives, campaigning for other candidates through her PAC, she can still impact the outcome of the nomination, etc.; I think she's considering the impact on her family. In short, I think she enjoys her current position much more than she seemed to enjoy campaigning in 2008 (according to the various accounts I've read).
I don't know if she won't run, because her actions and interviews send mixed signals (e.g., the bus tour began on the eastern seaboard, but then you have the documentary release in Iowa), but I'm inclined to think she won't.