Couple reasons I can think of:

There are the stressed out who don't have the time, energy or resources to smooth out the contradiction in their belief you just showed up and which is causing them pain. They can't deal with that pain and they sure as hell can't deal with the stress that turning their world-view downside-up would bring if you were correct.

Some (the redditor archetype) might have built a towering theoretical construct that supports the least painful option for them. They spent a lot of time perfecting it. If what you're saying contradicts one of their conclusions it's obviously wrong, they spent a lot of time building that tower and therefore know more than you. The more of their tower it would topple the wronger it must be. It can be dismissed without further examination. Your insistence on it just proves how stupid you are, do you not know how long they spent researching from the conclusion backwards? God stfu already.

Then there are the naive and (perhaps willfully) uneducated whose whole belief system legitimately is blue=good. And thats it. It's very nice and simple. The reps are obviously hateful and evil and the dems oppose them so they must be good. Look at how they talk about racial equity and wealth imbalance, the more I hear them talk the more nice things they say. Well this was an open-and-shut case. But if blue is good then people opposing them are either ignorant or bad. And if you are actively opposing good even after it was explained to you that your actions will lead to the bad team winning, well you must be evil. Another open-and-shut case, thank god the bad team makes it so easy to see they're bad and whose side I should be on.