On top of the other explanations, it’s natural that many, if not most, who decide to check out alternatives don’t stick around for various reasons.

• They might not have found the right instance for them (or even realized they were supposed to).
• They might not care enough about the new state of reddit to leave, after all.
• The communities that kept them on reddit in the first place may not exist here so they have no incentive to stick around.
• The bugs, growing pains, quirks, and rough UX might have outweighed perceived benefits.
• They may have been put off by the model or culture for whatever personal/ideological reasons.
• They might still be using fediverse platforms but isolated by fediblocks or by their own choice.

They may or may not reconsider in the future, or their usage of the internet may have changed entirely (so they’re out of the game, so to speak).

We should just keep doing what we think is best for the kind of communities we want to see emerge and thrive here. Growth for its own sake is not helpful or valuable.