As a Senator she has it made. There is no need to move to the Executive Branch and give up her fiefdom for life. She can move up to a leadership position in the Senate eventually and be heard every day throughout the world as well as having more control over the Federal budget than the President himself has. True national political power resides in the Senate.
I don't think Hillary wants to stay in the Senate. A lot of Senate work is boring. She has always wanted to be President, and her Senate seat was just the first step.