Its because the subjects that migrated better to lemmy are tech or tech adjacent communities, as these people are more willing to jump ship to another platform than less technicly inclined communities are.

I see it as less of a specilization and more about the userbase.

Its why you get communities like linux, gaming, tech, android, apple, hell even piracy doing well in lemmy because they all kinda have one thing in common, a statistically higher chance they are into tech. The more casual a community, the less likely they are willing to make the plunge onto lemmy.