The same reason people hate leftists, feminists, trans athletes, "gamer girls", people on welfare, blacks, etc. An image the right cultivated of the group, out of convenient easily-hateable annoying people in it that they could use to create a generalization/stereotype out of. It's something that's able to happen to any group, I could portray any hobbyist or activist in this way the same exact way as these "annoying" groups are portrayed, but the right is particularly willing to just flat out lie, slander, and cheat their way into making countercultural/anti-status-quo groups look as absurd as possible, to the point that the majority of the population falls for it (even those that don't consider themselves to be conservative).

I'll make a comparison. Conservative/"anti-sjw" thumbnails often have a picture of some angry-looking rainbow haired woman, usually the same few, in order to be like "look how irrational and crazy these feminazis are, she must hate men so much" and like 4 out of 5 of those times it's a picture of a woman that was protesting a literal neo-nazi gathering or something, not some sort of radical crazy man-hating feminist. But the internet has conditioned the average person to look at someone like that and immediately think they're an irrational "feminazi", and conservatives showing these pictures everywhere and making 100 videos on the same person makes people subconsciously believe they're rampant and have a massive (and bad) grip on society.

Same kind of thing happens with vegans, you have the same 10 or so internet vegans people use to portray veganism that conditions people to think poorly of the concept "vegan", and when these influencers are confronted about it they say "I don't hate veganism, I just hate the annoying vegans" then they go onto Twitter to complain about the vegans and how they're irrational for not eating meat and their brains must be de-evolving or something. They know what they're doing, but they can hide behind plausible deniability, and the majority of viewers fall for it.