It’s because everything that becomes popular starts with people in the 14-20 year range.
Myspace, Facebook, Digg, Reddit, Instagram, Vine, Snapchat, Tiktok…
The Fediverse will eventually grow once something triggers a snowball effect due the nature of connectivity and federation. Until then, the way to make it grow is to have young people make it popular.
Edit: There also needs to be more “fun” here. I think the Fediverse is a more “serious” place which isn’t what those other social media sites offered. “Serious 24/7” isn’t that enticing for young people.
The other day I was imagining a game or game service that has you make a Fediverse account of some kind. You get people to signup with a username that automatically creates a “@fedi.games” account. That account can have access to Mastodon, Lemmy, Pixelfed etc.