Mastodon needs to absorb a critical mass of the users who drive content on Twitter in order to be a viable replacement. A Lemmy community only needs enough members to keep itself fairly active.

I think there’s a fear that if a niche subreddit goes private permanently that community will be lost unless there’s an analogue set up here.

There’s going to be a learning curve, and I’d imagine that specific instances will eventually pop up as the user base grows in size and grows more comfortable with the platform.