You know, I'm pretty curious, and I feel like newcomer speculation is a fine enough place to discuss this, given it's related to one of the most heavily-requested and speculated newcomers in the Smash bubble (even if not in general). I've seen this for other characters, but I don't think I've seen it to the same magnitude as I have for this character.
Why do people vehemently hate Geno and his fans so much?
I would say 'hate' in this case would be defined as, "I hate him and I don't want this character in, even if it'll make some people happy, and I judge whoever likes this character and/or wants them in Smash." I wouldn't say hate as in, "I personally don't like the character or am otherwise indifferent." Hate is too strong a word for that.
I'm not saying this to point fingers, especially since I actually used to be very exasperated with Geno myself before I ended up loving the character. I'm frankly curious as to why people seem to, when they mention dislike of characters people want in Smash, especially secrete venom when it comes to Geno. Is it because of his irrelevance and side character factor? I understand that, and I completely agree that this is a problem, but why would it go as far as to that much hatred and spite? Is it bad experience with his fans? Is it because you want a different character that Geno might 'compete' with? Is it because you find him overrated?
I'm not asking this to be like, "Why would you? He's GREAT!" or for any desire to convince anyone to think otherwise. I'm honest to goodness just fascinated by the hate base he has, and I would be interested in more insight. It's kind of neat to me. Geno is easily one of the most polarizing figures in Smash speculation. The discussion this could bring up would likely be an eye-opening look into how Smash fandom behaves, and I'm such a junkie for getting into the social psychology of fandom. That said, I do recommend not passing judgment on people just because of a video game character they like, but rather, how they behave about it. I think that's the most mature way to handle that. That's about as much bias as I'm willing to dole out on the matter, though.