I think of this as analogous to the movement to get women to wear pants. It’s not that we wanted them to present as masculine; it’s that we wanted pants to stop being masculine and start being just clothes. Basically all masculine coded attire became androgynous, but almost no feminine code attire did.
It’s not like skirts are inherently feminine either. There are plenty of examples across cultures and time of it being perfectly normal for men to wear them.
He represents Vermont, our second least populous state, with only about 650,000 residents, and 371,000 votes cast in the 2020 election.
For reference, Washington DC has a population of 690,000, and cast 346,000 votes.
Additionally, Sanders is a long time incumbent. He was elected to the House of Representatives in 1991 where he served until becoming a senator in 2008 (which was prior to citizens United). In Vermont, the house of Representatives is a statewide election due to their small population.
Despite being an independent, Sanders has in many ways been acting as a Democrat, so the Democrats have avoided running against him for fear of splitting the vote. Combined with the overall democratic lean of the state, that gives him a relatively secure position.
Many other Democrats are similarly secure in their own seat. However, as part of being members of the party proper, they are under much more pressure to raise money for the party as a whole to spend on competitive raises