it's been better marketed, and people struggle with the concept of federation and picking a server. and I guess the invite-only, artificial exclusivity strat has actually paid off for them initially, unlike for Google+.
also, a matter of culture. I've seen many newcomers complain about how some long time users act as HOA, reminding everyone to act according to the long-standing rules. many people of colour have experience many forms of racist behaviour, too, which has driven some communities away.
oh, and the federation/defederation business sometimes gets way too messy, which [cynic mode on] makes it difficult for people who want their Personal Brand™ to gain as many followers as possible over the entire network.
Mastodon relies on users setting the language their post is in manually, so if someone posts in two languages and forgets to switch between them, they don't get filtered out. I know there are some other pieces of software that switch it automatically, I'm fairly sure Calckey automatically recognized the language you were writing in.
I already toggle the Mastodon settings to ensure that I got the feeds to the language I want (I want English only) and I still got feeds on different languages such as German and French
again, if the person who is making the post doesn't change the setting, it won't get filtered. if they type a message in German but the post's language option is set to English (which I think is the default on some major instances), it won't get filtered out.
you can usually check what their post's setting is by starting writing a reply to them, as the language option of your post will switch to the one they post in.
That is why Mastodon is hard to get mainstream because not everyone wants extra effort to do this. Even, Twitter, threads and bluesky much better to filter their language content