It’s multiple factors but boils down to them being easier for non-techie people to access. It’s also a culture issue. Bluesky culture is more like Mastodon than not but it is more diverse and shifts younger. Lots of gatekeepers and harassers ruined it for would be mastodon users particularly Black and Brown folk that were harassed out of the space. One of the biggest minority ran instances was shut down due to overwhelming harassment. People also don’t want to be preached to nor want to be told how to use their own damn social account and be told they’re using it wrong. Most people legit just wanted Twitter without Musk.