Because sheβs one of the most toxic and inept politicians Iβve ever seen and played a bigger role than anyone outside the Biden family in getting Donald Trump elected. She consistently underperforms other Democrats in her home state and nationally in polling, and her numbers get worse as you go further into communities Democrats need to win. Nobody dislikes Elizabeth Warren less than Democrat and D-lean unaffiliated voters in her own state.
Not surprisingly then, her influence on the rest of the party has been a disaster. More than any other single person she is responsible for alienating tech from the Democratic establishment (not exclusively but in particular with her absurd war on bitcoin), and that played a decisive role in Trumpβs victory in November.
I never thought we would see a politician more disinterested in the reality of electoral calculus than Hillary Clinton, but Warren managed to surprise me.