maybe is because of their “about” page? (i just searched for “sonomu mastodon” in duckduckgo, but even there you can see a “create account” button):

SoNoMu = Sound Noise Music

We are a mastodon instance for musicians, sound-artists, producers of any kind of aural noise, songwriters, bedroom producers, sonic manglers and algorave livecoders.

Please do not ask for account here just to try out Mastodon. Our resources are scarce and we’d like a close-knit community of fediverse-active music-makers who are already familiar with fediquette and in need of safe space that they can and know how to use.

When you request a new account on our instance, make sure you introduce yourself thoroughly - as a sound-maker, AND as user of fediverse. Again, please don’t ask for an account here if you don’t know what are content warnings (CWs) and ALT image/media descriptions for within the fedi/masto culture and you are annoyed at people trying to make safer and better spaces for those less privileged.

We take moderation seriously.