As someone who used to be vehemently anti-lemmy, it's a few different reasons.
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It's something new. Honestly is as simple as that. Most redditors are straight up threatened by new features, new looks, new anything. New Reddit is an example of that. To be fair it is hideous but it's also drastically underused according to reddits own metrics. This just stays consistently with everything. People prefer old subs to new, prefer old users to new, old memes to new. Why? Dunno. Could be as simple as just that they know it so it's comforting.
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The propaganda that reddit put up against Lemmy was pretty insane. The first few mini-migrations set people up with weird expectations and a lot of them bounced back to reddit with weird notions. Some of it was based on shitty admins or shitty servers (cough lemmy.ml cough) but other things seemed to be almost coordinated against Lemmy. By the time that the big migration from Reddit killing off third party apps/API use a lot of people had heard one or two things and just started spreading it. Redditors often don't source material and just kinda spread rumors or 'feelings' or upvote one idiot who seems like he knows what he's talking about while blatantly lying. This has never gone away. The same idiots keep whining and being dismissive.
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Redditors are hateful. Not purely hateful people or anything but the atmosphere encourages hate and division. I still browse reddit occasionally and I'll check the comments out about a post. It's always so bitter and angry, snapping out at one another. When every crab in the bucket is pulling you down, you get stuck in that habit too. Until you break free of reddit you don't realize just how bitter it's making you. Lemmy doesn't have those vibes and it can be really off putting to someone still in that bitterness. Kindness and people getting along almost comes off as stupid and naive so you just kinda dismiss the entirety of Lemmy as a whole.
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This is a conspiracy but I'm positive that Reddit admins are purging a lot of references to Lemmy that don't show the site in a positive light. When the API shit was happening people kept pointing out that certain communities that were supportive of Lemmy suddenly got locked behind a NSFW curtain that forced users to be logged in to read the community. A lot of people talked about how certain posts and stuff were being removed, especially ones critical of Spez. I don't think they stopped that campaign and I think they still try to demonize the hell out of Lemmy. Could be because China has a significant hand in reddit now or it could be because Spez has a tiny dick and a tinier ego. Dunno. But I think they're weighting the scales.