Because it is harder to get people I know on board to mastodon than to bluesky. I don't want too many random people on my feed. Most of my feed consists of people I do know and actually interact with. Mastodon doesn't have as much shitposting as I would like and takes awhile for it to get situated to the point I ended not using it. Why bother going to mastodon when I have my friends on bluesky.