I've been asking people around me what is holding them from using Mastodon (even after some of them joined), and I got 4 reasons:
1) Not many people here
2) Can't find people here even if they're here
3) It looks very complicated, especially the instances thing
4) Doesn't seem too different from Birdsite so why bother
For people to adopt this platform, these four points need to be addressed. And that too in ultra-simple non-nerdy language.
I'm relatively new to mastodon myself, and I can relate to point 1 and 2 for sure.
As for point 3: This is a hard one for me. As a fairly technical person the concept wasn't hard for me to grasp, but picking an instance took some time. I can see how, with centralised services being the status quo, mastodon and it's multiple instances can be very confusing at first. Having these multiple instances is one of mastodon's key features tough, so it can't (and shouldn't) be avoided.
Initially I used https://instances.social/ to look for instances that align with my interests, but I quickly realized that I didn't want to be in a bubble too much. So when I saw mastodon.nl in the list, which is an instance for people who live in the Netherlands, I figured that that would be perfect for me: Some engagement with fellow countrymen on the local feed, and the entire fediverse to explore and find things that interest me on the federated feed.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!