When I joined Mastodon in 2017, I picked mastodon.social because it was one of the larger instances, with good uptime, and I wanted one that would be likely to stick around.
Fast forward 5 years, and the sudden growth in user numbers this year (thanks Elon!) has resulted in it becoming problematically large. There have been issues with moderation, leading to some other instances defederating from it.
It's also not good for the Fediverse to have a few huge servers. So…
