Short-ish version:
1) There's a list of really bad Mastodon instances that only exist to host CSAM and launch attacks on Black folk, brown folk, Jewish folk, women, LGBTQIA folk, and other marginalized groups. It's a full time job keeping up with new additions to that list.
2) Some experienced admins do this work to block them, but most new admins don't. There are now 26K instances, most of which have new-ish admins that don't block this "worst of the worst" list.