Those who didn’t vote, or voted third party, due to the pointless war in the middle east that involved war crimes just like every war I can think of since the Geneva Convention became a thing, that President Biden funded, did so in safe states that VP Harris won.
The makeup of the United States means that Republicans have an advantage in the Senate and therefore also the Electoral College.
Republicans gerrymander, Democrats half-heartedly gerrymander, since that is against the ideology of liberalism. This gives Republicans an edge in the House of Representatives as well.
The Republican advantage in the Senate is so great that the only way for Democrats to get a majority is to include neoliberal or conservative senators like Manchin, meaning progress is continuously stifled.
The Republicans are allowed to get away with stretching the rules, while the Democrats have to follow the rules at all times. Part of this, again, is due to adhering to liberal ideology, and part of it is due to the ruling class favoring Republicans. There has been a conservative majority in the Supreme Court since the 1980s. Democrats are controlled opposition, in that no matter how hard they try, they will never be able to enact meaningful change.
An actual left-wing candidate would not be liberal, as is the point of this post. Therefore, they would have no chance of winning the Democratic primary. That would force them to run as an independent or in a third party, and our system makes it almost impossible for a third party candidate to win, at least at the national level.

Yes, it is better to vote for a Democrat than a Republican, but it is much better to build grassroots support for leftism, which, shocker, is what leftists have been trying to do in the US for centuries. If anything, the leftists are doing the most to fight fascism, by trying to get rid of the US system of government that is biased towards the status quo, which by definition benefits the ruling class.