One possibility...
She really doesn't want to garner lots and lots of public attention and spotlight on herself and her "persona", but she did have to announce something due to the exploratory committee thingy. The reason being, she comes across poorly.
The more people get a true "feel" for what she is like, the less they like the thought of her being President of the United States of America.
She would rather fly under the radar, to the extent that's possible, and just trade off her being a woman, a Clinton, etc.
Hmmmmm?
I wonder how the peoples of New York feel about her now?
She hasn't even spent a full month in her 2nd term as Senator, and she is already making plans for her next gig.
[No surprise, of course.]
Makes sense.