I guess the communities have to be of certain size to function and to feel welcoming to post into. For the first point you definitely need enough active users to make it feel alive but the second point is probably very person dependent. To me commenting in the big subreddits felt to much like showting in a very crowded space (so I didn’t comment much) while currently on Lemmy they feel more comfortably sized and somehow more real.

Perhaps for the same reason I never really “got” twitter. I understand it’s usefulness for journalists or celebrities but for me it was too close to screaming into the void to be useful/comfortable.

As for Reddit, many people will probably stick to it simply through the force of habit.