The truth is simpler than the conspiracy: they're not on the platform because that's not where there peers are, and there's no advertising or other evangelism constantly telling them they'll miss out if they aren't there.
And - that's fine. I have to admit, I'm very confused by the attitude that if everyone isn't on mastodon it's somehow a failure, or indeed that we even want everyone here. Big crowds bring trouble with them. We left a lot of those people behind on purpose. Is that condescending? Then fine - we condescend. The world isn't made for you, I hope you find somewhere else you are happy.