@Epsilon It's not, but it's the one many first-time Fediverse users coming across from Twitter end up on. (Much like how many users coming across from Reddit end up on Lemmy first.)
Then the jump from Mastodon to Calckey/Pleroma/Frienica/Pixelfed/etc in many cases comes later.
From what I can find, Threads has 10 million active users, and Mastodon only has 1.7 million. Threads may have been a "massive flop" in that they're not doing as well as hoped and the userbase is declining, but it's still far more popular than Mastodon, which also has a declining userbase outside of some recent spikes.