She can remain true to herself by locking key parts of her agenda in with Biden. I mean, if we want to be cynical about it, if Sanders does win she gets everything she wants anyway, so why take the risk?
Also I think she is actually midway between Biden and Sanders ideologically. Let's not forget her "I am a Capitalist" speech. She is a hero on consumer reforms that benefit the middle class, and that is great, but she is no proletariat revolutionary. If she can lock Biden in to her consumer protection, credit, and student loan policies she's done more than a good day's work.
It is very, very strange to me that she didn't take off. She seems like the perfect mean between Biden and Sanders. What's the explanation? Is it really just pure sexism? People can't stand a determined woman who is light years smarter than they are? Are the masses really that awful?