> Sure, but why Instagram specifically?

Because Instagram is specifically the place that those people already are. It really isn't hard to understand why others join it. They weren't even first, just the place where people ended up being, until they won't be there anymore and instead it'll be another place.

Also matters what niche/community you're a part of, some of them already left Instagram for other places (like furries seems to have gone to Mastodon/Bluesky as far as I can tell [but not a furry, so please don't sue me if I'm wrong]) while others are still there, like most musicians/artists still seem to prefer Instagram, even non-mainstream ones.