She's angling to be his running mate.

This is what was explained to me when I was pissed about her endorsing Creepy Joe:

When entering the primary you have to sign a contract stipulating that you'll play by the rules, which includes backing the eventual nominee. She repeatedly gave her word that she'd endorse and support the eventual nominee.

Had she endorsed Bernie upon suspending her campaign that would have been taken as a smear of Biden, and it would have been used by the Trump campaign, and really sealed her fate of forever being a pariah to the Democratic party.

She didn't have the budget to continue especially when they kept changing the rules to keep her out of everything.