The public seems to like Hillary a lot better when she's far removed from the presidency. The closer she gets the more distrust she evokes. When she ran in 2008, her popularity sagged. That's how she managed to lose a nomination that most people assumed was hers for the taking.
She is easier for people to take in the role of diplomat than politician. When she lectures dictators at the UN, voters tend to approve. When she lectures audiences in Iowa, they tend to bristle.
Being secretary of state lets her look serious and diligent, which she is, while sparing her from close daily scrutiny. Running for president would put her back under the microscope, where she doesn't look so appealing.
There are other things working against her. One is that it's very hard for a party to win three consecutive presidential elections. Except for 1988, it hasn't happened since Harry Truman's day. The Clinton good times, remember, were not enough to deliver Gore to the White House.
Nor is it clear that Hillary is such a great candidate. Maybe she learned invaluable lessons from the last try. But then, so did Mitt Romney. Alas, the same shortcomings that kept him from getting the nomination that time kept him from winning the election this time.
We had a chance to elect Hillary president in 2008, in a year made to order for her, and it didn't come to pass. Don't be surprised if it never does.