A while back, Reddit made some controversial changes to their API pricing scheme (going from "free" to "outrageously expensive and clearly meant to just kill competition") resulting in almost all third-party Reddit browsers to have to close up shop. This included Apollo, my personal favorite.
The fallout from this change blew fresh wind into the sails of the maintainers of Lemmy, and brought a surge of users to all federated platforms. (Elon's acquisition of Twitter and subsequent rename to X also had this same effect, but for Mastodon and other micro-blog software). The promise of a decentralized and federated system that otherwise feels the same as the walled garden of Reddit is tantalizing, after all.
"decentralization" and "federation"
I'm not going to waste much time on explaining these - I'm definitely not the right person to explain it correctly so take these definitions with a grain of salt.
Decentralization: where a service isn't hosted by one person or corporation, but instead hosted by many. Torrents, for example, are decentralized.