Thibaultmol As sarno already said, I think the usability of Bluesky is a major factor.
Most people won't really care that it isn't completely open as long as it is easy to use, and has a good userbase. I think this is partly why Mastodon doesn't have as many users.
I do however wish that Bluesky would open up even more.
Luckily for us, Kagi has accounts on multiple social media platforms, so none of us are left out 🙂