I think she finds she has more power as Secretary of State. The Clintons have always been about under the table back door deals, and in State she can go around the world setting up deals for the USA, her real friends, and herself.
As president she’d have lots of people watching her every move. Of all the Secretaries of State I can remember, Clinton is in the news the least. When she goes to Myanamar, then Angola, then Northern Ireland, what is she doing?
At her confirmation hearing, when someone in the Senate mentioned that they were uncomfortable with the foreign money pouring into the Clinton Library fund, she said, OK, the money will stop coming into that fund!
You're probably right, but my gut tells me that Hillary really has had enough of politics, and wants to finally ease back into the life of a private citizen. She's been a part of the Democrat political machine since she was First Lady of Arkansas.
Her heyday is long behind her now, and she (at least) seems aware of that fact, unlike so many in the chattering class (and on FR) who simply refuse to believe that she doesn't have some secret plan to TAKE OVER THE WORLD!!!!!