When it was turned into one by politicians and activists, just like masks were.
I'm a firm believer that if you actually sit a few people down and try to have a sane conversation where people actually talk and don't spout slogans, you find that their views aren't all that different, then just put emphasis and importance on slightly different things, and have been presented with subsets of the facts that emphasize those preferences.
The problem is, we're going on a few decades of a concerted effort to confuse and separate us based on language, and specifically meaning of words. What used to be camps choosing words that placed their views in a beneficial light (pro-life vs pro-choice, for example), has moved to very purposefully continuing to use the same words on both sides but to re-frame them to mean something entirely different than intended by that side. For example, Black Lives Matter and how it literally means different things to different people, regardless of the originators intentions and protestations.
Information silos allow this to happen. If you can exert control over the information environment, you can change the definition of words, and then people can't even communicate correctly any more. That's a real goal of those that wield political power. Moderate is just another name for someone dangerously close to being a swing voter to them, so promoting enough animosity that even things overtly against your own interest are seen as the lesser of two evils is a winning strategy.
Lest you think I'm talking about the left or the right, they both do it. And lest you think "sure, but one side does it more", no, they both do it so egregiously that the difference does not matter, and all you're seeing is the effect of Knoll's law in action. Make no mistake about it, you're getting screwed from both sides. That you tell yourself that your side is the more sane one is just the fiction that lets you sleep at night (and hopefully that's all it's doing, and not empowering you to be a complete asshole to people just as screwed over as you are).