There's a few technical and non technical reasons someone might be on Bluesky/AT instead of Activity Pub. Protocol specific there's:

) Account ownership (theoretically at least, migration is still in development). Though it's hidden behind domain based identification there's a cryptographic key that let's you migrate to another PDS even if yours is down or banned you.

) Performance. Hosting something like a PDS is lighter than an Activity Pub instance.

) User level configuration. Bluesky let's you set custom moderation lists and algorithms, something you can't on Activity Pub.

) Compatibility. Building something like a link aggregator on AT that is compatible with a microblogging platform like Bluesky is likely a lot easier then Activity Pub since AT is broken up into PDSs and Relays. (To be fair compatibility does work on Activity Pub, but it's got jank).

There's also some less technical reasons as well:

) Bluesky is a platform and you don't need to learn a protocol to use it. Yeah it's not that hard to learn how any of the big three protocols work, but it's also not that hard to change your car's oil or sew ripped cloths instead of replacing them - but how many people do those? I'd guess 80% of Lemmy is an IT guy between 20-45 so it can get a little echo chambery on how easy tech is. One if the reasons Threads makes up 99.5%+ of the fediverse.

) Defederatiation is becoming a mess. If some random Joe has a friend on Bluesky & Nostr (both bridged), a few on threads, and a few spread across different instances; yet he can't reach all but 1 or 2 of them from the instance he chose to join on joinmastodon it might be time to reconsider how things are done. Techy people might have no problem sifting through a long list of servers to find the right one, but somebody who's already on the fence is probably going to quit at that point.

) Bluesky has a more mainstream culture, while the fediverse has very specific thoughts and ideas. Had I said I was on Windows you all might have put a hit out on me 😆