1) Better (optional) algorithmic feeds. Mastodon's "explore" is weaker than Bluesky's "popular with friends" and "discover"
2) Quote-tweets.
3) Easier onboarding. Mastodon forces you to care about which server you're on and it does matter and migrating later is hard. Meanwhile, BSky has "starter packs" that people can produce for each other with lists of users to follow to easily jump into a community.
4) Username-as-domain is better than the Mastodon "confirmed links in profile" thing for self-verified accounts.
I wish the properly-federated OSS community-funded one had won but I'll take either to be done with Twitter.