So when is Elizabeth going to throw her weight behind a candidate? When it doesn’t matter any longer?
I swear, this lady can’t get out of her own way.
Why does she have to endorse anyone?
Politics.
I doubt it really matters at this point.
It matters. More to Sanders than to Biden, but it definitely matters. Less and less for her the longer she postpones whatever deal Biden no doubt already offered her, however. Which likely means that Biden didn't offer her VP, which is what she would be holding out for.
With Biden set to win Michigan, her endorsement will simply be a fait accompli, so she may be waiting for Sanders to lose in order to work out a VP ticket with him and/or threaten Biden.
If Sanders loses Michigan then he SHOULD get the fuck out of the race, just like he should have done in 2015 just later in the month. Instead he pointlessly fed his ego and kept going and fucked us all with Trump. He's probably egotistical enough to do it again, only he'd need Warren on board as his VP to justify it a second time and she no doubt understands this.
So my guess is that's her play. If Biden wins Michigan and Sanders decides not to get out like he should, then Warren will make a last ditch call to Biden's camp saying she'll endorse Sanders against Biden unless they give her VP. Biden's camp would know that could mean another toxic zombie civil war primary.
It won't matter to her future if she goes down with Sanders, unless she goes as toxic as Sanders would go, but she can always pull the plug on that if she goes all-in and then really be the hero for the next available general run.
And if another miracle happens and Sanders wins Michigan like he improbably did last time, then he definitely won't get out like he still should and a Sanders/Warren ticket would look even more promising to him.
ETA: I should add that Sanders most likely already has offered her the VP ticket, but she's holding out because she actually wants a chance at being VP and that's only going to happen with Biden. So for her, it's about a real chance to be the first female VP or tie herself to a sinking ship that could springboard her to being first female P (maybe) some time down the line.