Easier sign up. On BlueSky you can just sign up for an account and go. You don't have to worry about picking an instance or anything like on Mastodon, which can be a bit off-putting for someone not familiar with federation.
Onboarding to Mastodon is actually identical to BS/Threads now. They've made huge improvements. It's a shame that most of the news media's experience discovering Mastodon for the first time was in Oct '22 because it's left a bit of a "techie" aura around the whole thing they're still trying to shake off. If Mastodon was then where it is now, I don't even think BS or Threads would try to compete.
FYI- Signing up and following people on the Mastodon app now is literally just as easy as it is on BS. All the Federation stuff is hidden unless you want to look for it. It's very nice.