A lot of subreddits moved to Lemmy.world so in turn I think a lot of the reddit migration ended up here (myself included). Mastodon.world already had a large user base so it probably just made sense to rely on their new lemmy instance to handle the new users. Other instances like lemmy.ml sort of froze new signups at the time so that also funneled more users this way.

It was the first to throw some real money behind it's server infrastructure. A month ago most Lemmy instances - even the "big" ones ran on $10 a month 2 vCPU VPSs.

There was another influx early in June when the API changes were announced but before the blackouts even happened.

Most instances with public signups started struggling but Lemmy.world launched and Ruud upgraded the server hardware almost daily to keep up.

Other instances even had to pause signups from time to time but world kept working and so gobbled up most signups on those days.

Once it got to be a top 3 instance and then top 2 and then first it became a self sustaining snowball as a lot of people chose the biggest instance by default.

Being general-purpose, having easy sign-up without needing an "application" or manual approval, and having decent reliability/uptime.