Mastodon is gatekept to hell and back, the technicalities of federation are exposed to the user for some reason (you already lose half your potential user base right there), infighting between instances means that you won't see the entire discourse of a post depending on which instance you're at...
And besides all that, bsky is not as "corpo" as mastodon fanboys make it out to be. They're on track to open up to privately hosted instances as well, and you can already run most of their backend stuff yourself.
I assume the main reason is that ActivityPub is a mess and quite overcomplicated for bsky's needs. Being permanently tied to it seems like a big risk. There's no reason why they couldn't make a compatibility layer later and hook into it.