If I remember my Lemmy history right (someone correct me)....

In the early days of Lemmy, everyone was on one instance (lemmy.ml). The founding Lemmy developers (and their friends, I guess) were tankies. At some point they decided to make Lemmy more attractive to the general population, and make the flagship lemmy.ml less overtly Marxist-Leninist/offputting. So they split lemmy.ml into 2: lemmy.ml for non-commie stuff, and lemmygrad.ml for commie stuff. Since the bulk of Lemmy users in the early days were still tankies (or their friends), they were still generally using both instances very heavily.

The day that lemmygrad.ml ceases to be the #2 instance and becomes just another niche-interest instance is the day we can say Lemmy has truly become mainstream, I think.