@hawkeye10,
I didn't watch enough of Obama and Clinton to answer that capably, but rule #1 is the front runner doesn't stand on an equal footing with the challenger - it suddenly gives them gravitas or increased stature just to be there with you.
2. Bernie is dying for name recognition. She has it - he doesn't - polls show that as he gets it, her numbers drop and his rise. She knows he can't pay for airtime, so a debate is the biggest (free) chance he has to get his name and ideas out there.
3. As smart as she is, he is smarter.
4. His policies and her faults would rout her to hell.
She'll feign something to get out of it. And her followers are as corrupt as she is. She'll get away with it.