I see 4 possible reasons:
1. She’s saying FU to politics and the whole digusting business, that I’ve got a family to raise and a Governorship or the Presidency is less important than that. The latest personal attacks on her family would be ample reason.
2. She's had it with the some 180 frivolous lawsuits that have been filed against her and, like many of us, believe lifes too short.
3. She's going to take a serious run at the Presidency in 12 and feels she needs to put 100% of her efforts to that end.
4. There's some skeleton in her closet that will be imminently disclosed and she's getting out while the gettin's good. (least likely in my mind)