I thought it was because lemmy.ml closed registrations?
My original account is on lemmy.ml, but then I made this one on hexbear.net which now I use as my main, and I remember lemmy.ml was open long enough so I was able to create mine right before majority left reddit and then they were closed as it really spiked. I’m not sure if lemmy.ml registrations are open again but when most people were seeking to join Lemmy and lemmy.ml was closed then it seemed like lemmy.world was the default by default.