Federation works for Twitter, it doesn't work for Reddit. If my Mastodon instance federates with yours, I can see your posts, you can see mine, we're fully merged. If my lemmy instance federates with yours, we can browse each other's communities, but those communities themselves don't merge. The userbase merges but the communities do not, and the community is the base unit that gives Reddit value. Federated Reddits will never take off for this reason. You either have a bunch of fractured communities that are all worse because there is no network effect, or you have one instance (or one community) absorb all the traffic and become its own centralized fiefdom.