I moved from Lemmy.ml because I liked the name of Lemmy.world and it ran a newer Lemmy version which meant I could make communities. I moved from Lemmy.world because they defederated from piracy communities they didn't even host (but for some reason still kept the small piracy community they DID host) From thelemmy.club because I couldn't see the Hackintosh community from there (probably defederated) Now I spent some time looking on join-lemmy.org and checked out some instances and this (lemy.lol) instance seemed good, so I chose that.
Moderation and politics. Many in Lemmy don't like ML's stance on issues, including banning and moderating dissenting opinions. World won't defederate from Threads. Etc. Despite no major user difference, Lemmy is a much more "active" vs passive community in reinforcing their beliefs in what's right. It's why many migrated here to begin with.
Threads?
Meta's/Zuckerberg's microblogging service, also runs on Activity Pub (the protocol that allows instances of any given service to communicate with each other, basically the backbone protocol of the Fediverse).
Many Fediverse instances decided to defederate Threads because of who's behind it and because they're afraid Meta's ging to do to AP what Google did to XMPP.
To add: and because they don't want the content they create to become a money maker for a company that allows abusive organizations to stay online.