I think the main one is that Nostr is supposed to be really simple, and derive some security/privacy from that simplicity. Whereas ATProto and ActivityPub store and forward content so that each rely has a copy, on Nostr nodes the messages are all ephemeral. So it has something of a following with crypto bros and privacy enthusiasts. With nostr IDs are also based on public/private keys unlike ATProto/ActivityPub that use a username@instancename scheme, so signing and identifying content that belongs to a user is easier and more guaranteed to be correct.