Centralised moderation teams often lack the context to know what they're looking at. Fediverse admins each take care of a small, well-defined bit of overall moderation; the bit that affects accounts on their server. They know what's acceptable in their community.
"Mastodon users probably aren’t aware of CSAM on the platform unless it leaks into their federated timelines. This can happen when a fellow user on their instance follows an account posting CSAM. Ways to handle this problem are few. Though users who follow CSAM-disseminating accounts can be suspended from an instance by administrators, they can easily set up a new account on another..."