One reason it failed was because it looked like you needed a freaking PhD to understand the account setup process. It's difficult to convince people that your party's great if you make them solve a math problem correctly before you let them in.
Mastadon is awful. The learning curve and personal commitment is far too high for it to be a viable alternative. The same argument is found in why people don't choose Linux over Windows. People, especially journalists, love Twitter because it's easy.
Which mastodon instance was that? Some instance maintainers use questions or manual account review to stop spam. Mapstodon.space (https://mapstodon.space/auth/sign_up) asks what your interest in cartography and geospatial is. Maintainer said on a podcast that has cut 70% of signups. The remaining 30% are more relevant for the niche target the instance is looking for.