Because she's not officially running for office yet. Once she starts acting in both word and deed as a candidate, people will start thinking of her as "Candidate Hillary" rather than "Secretary Hillary", and her popularity will correspondingly go down (particularly any crossover appeal she has now).
That's already happened. Her favorability ratings haven't been sky high for quite a while now.
It'll happen some more.
Called it, although it happened even more than I thought, but she had Donald Trump to allow her to almost win.
