it’s a combination of factors. The software is not super robust, the feature set is not what people demand, it doesn’t have the financial needed, nor the developer quantity, and people really want an algorithm despite what a lot of people in the fediverse say. The problem is you have to consider what drives people to platforms like Twitter or Instagram in the first place, and a lot of those ideals do not directly mesh with the core fundamentals of your average FOSS enthusiast. It’s part of why I have become a big fan of blue sky is they are sort of tackling all sides at once. it’s free and open source, it has algorithms, it’s federated, It’s financially backed, it has a pretty big dev team, It is written in modern web development languages that a lot of other web developers also use. They just overall thought about it well and are executing efficiently. I’m not sure there’s any other platform that you could really consider jumping to instead of mastodon if you care about free and open source and you want to be where the people are.