I've used both for a while now. Here's some general reasons someone might prefer Bluesky:

• It has a better sorta algorithmic feed that does a decent job of showing you stuff you might be interested in compared to Mastodon
• It's presently not fragmented across a bunch of servers who may or may not federate with each other
• Other than getting the initial invite, it's way less convoluted to use/understand for the average person
• It looks and behaves more like Twitter. A lot of people just want Twitter without the Elon bullshit
• And in my specific case, the quality and volume of the high talent art community is way higher and that's what I want to be seeing a lot of on a Twitter-like website. I was on mastodon.art and it's largely amateurish stuff that clogs the instance and their moderation team is not for everyone to put it lightly.

I'm all for fediverse, but the fact of the matter is that Mastodon leaves a lot to be desired if you just were looking for a new home after Twitter. And to an extent that's just a matter of taste. A lot of people don't give a shit about the righteous Activity Pub mission, they just want an online town square where the people and things they want to follow are readily available and the vibe is generally friendly. Simple as.

I'm on both Mastodon and Bluesky, but I'd be lying if I said that Bluesky wasn't clearly a better experience for me at the moment given what I'd like out of a social media platform. I hope Mastodon improves over time and that changes, but it ain't there right now.