What thunder? Mastodon has had nearly a decade to go mainstream and it's still mostly tech enthusiasts explaining to their friends what an 'instance' is.

ActivityPub is fine if you enjoy your identity being held hostage by whatever random server admin decides to keep the lights on. Want to move servers? Hope you're cool with losing your followers. Want real account portability? Too bad. Want scalable search and flexible moderation? Also too bad.

ATproto wasn't built to compete with Mastodon out of pettiness, it was built because ActivityPub fundamentally cannot accomplish the task that ATProto/Bluesky is aiming for: a decentralized social network that isn't a cumbersome pain in the ass to use.