Because entrenched Democrats are under a ridiculous belief that everyone who isn’t voting for them is conservative. So if you spout “extreme” leftist ideals, you’re too scary to the people they are courting, which is conservative voters who aren’t Trumplicans.
I think this is what Morgan Freeman meant when he said he had a problem with Black History Month. Having a celebration of all that you are as a people is great, especially for barely recognized minorities, but we need to acknowledge that those minorities still exist and should still be celebrated even after the official festivities are over with.