Theres a few reasons! You can read some of the posts from miljan (npub16c0…6nvr) explaining their reasoning.
Main ones were:
1. Adult accounts (they don’t want those in the trending feed, understandable whether you agree or not)
2. Semisol was creating “fake” trending events by creating fake reactions/replies/zaps/etc so they blocked his npub.
Again, if you’re trying to build a functional “trending notes” view, these are reasonable steps to take not “censorship”.