Threads satiated the one thing Mastodon didn't - lazy asses who might have been curious but really didn't to make the transition to something that sounded unfamiliar, complex, and a bit crude to them. Using OS metaphors, Threads is Windows, Mastodon is Linux.
Chrome OS. Only taken seriously by corporations (who probably shouldn't be using it for official business anyway), intentionally de-featured and functionality limited out of some misguided 'vision' by management, and everyone else who uses it hates it.