I made several attempts to get onboard with Mastodon about a year ago when I stopped using Twitter. I used a tool to copy over as many people I follow on Twitter to Mastodon.
Technically, it's fine. The issue I have is that it felt exactly the same as Twitter, and I always disliked Twitter.
However, Twitter had reached a network effect threshold where most of the news in my industry was shared exclusively there, so I was... well not quite forced, but strongly compelled to use it.
Mastodon doesn't have this. It's just noise and thoughtless hot takes, much like Twitter, but without most of the news I used to use Twitter for. I hope it doesn't become another Twitter where I feel compelled to wade through the mess to get my industry news.
To sum up: for me the reason I don't use Mastodon is not technical. It's because it feels useless and I dislike it. I had hoped for a more thoughtful arena of public discourse, but Mastodon is not it. All the interesting tech aside, it's just a Twitter clone using different technology.