Threads was able to be pushed through other large commercial platforms, and onboarding was trivial for anyone coming through the portal.
Mastodon lacks that kind of exposure and the onboarding process of choosing an instance makes it feel more confusing.
As a result, Threads starts off with a network effect that is quite a bit more advanced than Mastodon’s. It has more high profile users that maintain an active account. It also claims to want to join the fediverse, which makes users think that they can easily onboard with Mastodon in the near future.