One problem may be that Sanders has no heir apparent.

[hack89]

36. She rejects both the Democratic and Republican parties

Sawant is a member of the Socialist Alternative party, the United States section of the British-based Trotskyist international organization the Committee for a Workers' International (CWI).[32][63] Sawant has stated that she does not advocate for any system like the "bureaucratic dictatorship" of the former Soviet Union, but for democratic socialism meaning "the society being run democratically in the interest of all working people on the planet, all children - everybody who has needs, and all that being done in an environmentally sustainable manner."[64]

Sawant said she rejects working with either the Democratic or the Republican party and advocates abandoning the two-party system.[8] She has called for "a movement to break the undemocratic power of big business and build a society that works for working people, not corporate profits—a democratic socialist society."[65]

https://en.wikipedia.org/wiki/Kshama_Sawant

She can only exist in a hot house political environment like Seattle. She has no national appeal or base.