Remember, AOC will be just barely old enough to be President come January 2025.
EDIT: Also Warren's lack of endorsement for Sanders (twice!) has made her anathema among certain segments of the left (the honest ones will still give her full credit for the CFPB, even if they're disillusioned with her now), Booker's killing a drug reimportation bill made him dead before he started (note: don't reimport drugs from us, we have cheaper drugs because we negotiate lower prices in exchange for lower supply, pass your own drug price negotiation bill), and as for the boxes Waters checks, Nina Turner checks those boxes and has been vocally pro-Sanders almost from the time he announced in 2016, unlike Waters.