I've been trying to sign up at lemmy.ml for days & it's not working.
In short, simply lemmy has been slowly growing by tiny amounts for a year... but it hasn't gotten nearly ready for major growth. Then reddit of course did the huge things causing lemmy to grow at 10x it's normal predicted amount, and because it's especially people from reddit, many of which don't really understand federation yet. They assumed joining the main instance is the way to do it. (rather than understanding that almost all instances can communicate with eachother).
In short it's a perfect storm for excessive growth on too small of a segment. on a network that wasn't prepared for the sudden increase in demand.