To be frank about it, the clips of porn needs instances to follow the steps needed for Section 230 to protect them (see EFF : User Generated Content and the Fediverse: A Legal Primer) otherwise it becomes a huge liability for every site admin who federates with such instances. I am not aware of any instances that have taken the step of registering a DMCA agent with the US Copyright Office.

Additionally, given the nature of the people commenting and the possibility of "oh, I'm 17, is that a problem" without the corresponding tooling for age verification and approved users, the moderation tools need to really get a focus for how to create them, implement them, and federate moderation events out reliably.

Furthermore, a significant amount of porn on Reddit is from amateurs who are trying to monetize or advertise. So far, the Lemmy user base has taken a dim view monetization (other than donations) or advertising. This makes it so that it's just not worth it for an amateur to need to maintain another social media presence that has little return.