Because people “just want it to work”.
from what I’ve read on bsky is that people don’t like Mastodon due to the instances. They don’t want to figure that out. Either because they’re too lazy or because they’ve spent so many years utilizing apps that “just work” that it’s beyond them.
That’s the meat and potatoes as to why Mastodon isn’t taking off, it’s “too much work” to use Mastodon.