AOC decided she was too important.
Or, perhpas, that Bernie wasn’t going to be the nominee, and she could more effectively raise her profile and promote her brand on her own time than as a Bernie surrogate (I think that word is especially apt here). For example, going on FOX for an interview with Brett Baier.
She endorsed him as a means to take up the mantle (once he lost) of the rabid leftist moonbats. Chica has Presidential aspirations herself.