The combination of users and communities in a single space.
On the regular fediverse, if there is an instance that doesn't moderate transphobia, I can just defederate my instance from it, because the only content on there will be generated by folk who either generate transphobia or are ok sharing an instance with people who generate transphobia.
On the threadiverse though, communities change that. Because there might be a trans friendly community, moderated by trans folk, whilst the instance itself doesn't moderate strongly for transphobia. If I defederate it, my users lose access to everyone on the threadiverse that uses that community, whatever instance they're from. Combine this with the generally underwhelming willingness of admins on the threadiverse to deal with transphobia (compared to the microblogging fediverse) and the end result is more transphobia on the threadiverse.
It would be different if the threadiverse had a large extant culture with strongly inclusive norms prior to the reddit migration, but it didn't, and the new lemmy mega instances that have popped brought reddit's vague tolerance of transphobia with them.