I think it’s a few reasons

• the communities are mostly all science/research focussed, so it’s got more of a recognizable culture and you know what to expect going in
• very engaged and curious people that post a lot of cool content

I like mander.xyz, and it’s a good place to build a community that fits that theme. I’m working on a few health/medicine communities started on Mander.xyz and people really helped get them going