Democratic political strategy
The democrats are still at their core a liberal party, and ultimately running a left candidate would be against their interests.
What’s really frustrating is the Dems just dont seem to have any vision of what they want. They clearly don’t want the dystopia of the Trump party, but aren’t really offering a vision of something different or a way things ought to be. (And they won’t be able to as long as they are trying to cater to workers as well as the Wall Street class at the same time.)
You are talking about “they” like the party is run by lizard people ruling by the divine right of kings. The “they” in the republican party also didn’t an obviously extreme right candidate and their base gave “them” the boot.