The Left's decades-long reliance on Political Correctness and Identity Politics made two massive errors:
First, it slowed the natural growth of minorities (particularly American Blacks) by lowering standards and expectations for them, shifting accountability for any negative behaviors from them to others, only enabling more of the negative behaviors.
Second, it created and nurtured a resentment in those who called out its weaknesses and counterproductive results, only to be met with screams of RACIST, primarily by white liberals who were defending their actions. Which, of course, led to the elections of 2016.
Hell, I warned about this for YEARS here. I just underestimated the intensity and depth of the resentment on the Right.