It used to be a joke. It used to be that conservatives and progressives had legitimate disagreements. Liberals tended to be scientist and academics, and that provided a certain kind of evidence that reality backed up their opinions, but a lot of the discussion concerned values and world-views rather than outright denial of reality.

It feels like conservatives leaned into it. "Well, if they're gonna rely on 'facts', let's just take that out of the equation". They started with evolution, where a majority of their base was already just-plain-wrong on the facts, and honed a toolkit where people look to each other and distrust anybody who tells them otherwise. Once they believe that the "mainstream media" is lying to them on one topic, they'll reject anything it says on any topic. At the same time they receive reinforcement that they've outsmarted the people trying to fool them.

That leaves us here, where the joke that reality has a liberal bias has become reality. Liberals are still often wrong on a lot of topics, but there's really nobody to argue with. They won't learn anything from arguing with conservatives, and when liberals argue among themselves, the division leads them to lose elections.