Because most people don't exactly want a community-led social platform that respects you and empowers user freedom, even if some say they do.
Get off your high horse. I work for a software company, regularly participate in beta testing and am very tech literate. Mastodon was agitating to use when I signed up and not intuitive. The community I signed up in also deleted my account during a “whoopsie”. A terrible experience drove me off with no desire to go back for such a tiny and relatively stagnate user base on an unstable platform. If that was my experience, the average person will absolutely not like Mastodon.
I picked kbin for no particular reason and then moved to lemmy. Kbin was a dumpster fire of spam and downtime. Seems to be permanently broken as of the last 2 months minimum