Selection bias. There’s plenty of overlap between the groups of people who know about it, care about it, use FOSS, use Lemmy etc. It’s basically a prominent characteristic of the stereotypical Lemmy user. We’re still a small and surprisingly homogenous group of people. If Lemmy ever grows like Mastodon, you’ll begin to see more diversity.
There’s also something you could call the “fish out of water” bias. If you’re not LGBT, you’ll suddenly notice how many LGBT people there are on Mastodon. If you’re not into ML, you’re going to notice the people who are.