Because the Mastodon community did the same thing we do every time there is a chance to get people away from corporations (e.g. Linux vs Windows).
People were looking for an alternative. The general consensus was it was hard to really grok federation. So, of course, The Community insisted on explaining federation and why it was good while basically only commenting on the instances that had closed applications. It was the equivalent of insisting someone who wanted to try Linux for gaming NEEDS to use arch and only needs to know twenty command line operations to get up and running.
So… everyone instead just went to Bluesky and Threads where sign-up links were provided rather than directory links and manifestos.
And… I am perfectly happy with that. Lemmy has a LOT of issues where so much of the community is talking about their ex-girlfriend (reddit) all the time and we basically get constant content and engagement farming that makes no fucking sense considering the userbase.
Whereas Mastodon actually IS a really good community that feels very different from twitter/bluesky/threads. It isn’t for everyone but I very regularly have genuinely good conversations with people in the town hall/microblog format. Whereas… I am not sure if I have ever had even a meaningful conversation on lemmy (whereas I’ve probably had maybe ten on reddit over the years?).