Why not participating in Mastodon? Because nobody really wanted a Twitter clone and the project is losing its momentum. Lemmy is 1000x better even if the total amount of users is less in absolute numbers.
Why using Bonfire? The first thing that comes up to me from the website is that this model of community-focused development seems more resilient to the wave of AI slop. A small Mastodon instance with 30-40 active people and limited federation would be useless. A Bonfire instance with the same people where you can work on community projects or scientific projects, sounds a lot more viable while keeping the shields up against the slop.