I would agree when Mastodon would it not already make it so easy for normies to join. If you can make a mail account, you can make a mastodon account.
It's not entirely true. I mean, it's easy to create account on any instance. It's not easy to understand the concept of fediverse, be "discoverable" as an artist or sometimes even following someone on another instance. That's why normies would pick something like threads or twitter, and the artists would have to choose the services that are more popular.
The question though, why won't they use mastodon alongside other services. And to help them use mastodon, developers could create tools that would make it easier to post and read to/from multiple services at the same time, for example twitter + mastodon + threads. But it won't be easy now to create such an app, because of twitter api costs, so probably won't happen. Managing another service without big payoff might not be worth their while. Also, it's what I assume would be the issue, so maybe I'm wrong, not an artist myself
Offtopic: I'm on meow.social, if you want to test DMs you can try DMing me @Crazy Pony. My profile: https://meow.social/@Mugox