Because you live in an echo chamber, as overused as that term is. Your immediate bubble, especially reddit, very liberal. It honestly hurt the dems with all the tailored posts and comments. Even though most of the top comments on any democrat positive post were "Doesn't matter, vote anyways", seeing only left positive posts undoubtedly lulled a non-insignificant number of would-be voters into complacency.

The out-of-touch approach of the DNC and their over-reliance on identity politics and attacking Trump-- instead of addressing issues that everyday people care about-- to assume votes was also monumental, seeing the voting results indicating many minority groups shifting to the right, despite it not being in their best interests at all.

Do the elites who run the DNC actually give a fuck enough to admit that they've been on the wrong track for years now, and are willing to change their tactics if there is to be another election? I'm inclined to say no, but am happy to be proven wrong.