Part of it is just the network effect. If the people they want to follow are on twitter then they do not really have a choice. Also part of it is the algorithms. For some needing to manually select communities or individuals is an inconvenience. Finally I feel like fedi communities have a very distinct atmosphere simply because very few people use it. This can all change in the future but the majority of the issues stem from just not having enough creators and users as well as the additional effort required to use these platforms.