Threads will only talk to Mastodon et al. for as long as the protocol remains compatible and servers allow it to federate. My guess is that one or the other of those things won't be true for long.
If your real question is, why do so many people like Threads better than the Fediverse? Well, the answer is that most people don't want to think about how to sign up and find stuff—they just want to click a button and have all their decisions made for them. Federation is not a selling point for the average non-technical user who just wants stuff to be easy. And that leads to two more questions.
First of all, does the Fediverse want to attract these non-technical users?
If the answer to that is "yes", how can they be encouraged to join up and stick around? The answer to that is, hide federation, which requires a few things. First of all, offer a default server on sign-up (to avoid overloading a single node, this should be chosen randomly from a list of general-purpose instances that are uncongested at sign-up time, as someone else already suggested). Making it easier to migrate an account between servers would help a lot.
The other thing is that there needs to be some kind of usable list of things to follow—people, hashtags, lemmy and kbin communities, or whatever is pertinent to that corner of the Fediverse—built into the interface, not off somewhere on a third-party site whose lists may include things on a server yours isn't federated with, and offering the ability to subscribe without doing any copy-and-pasting. Ideally this directory should offer more methods than just keyword search or scanning everything on the list one item at a time for discoverability.
For Lemmy, these things could all be done through changes to join-lemmy.org and the default front end, plus some elbow grease to create and populate a browsable community index arranged by topic, in a form that can be easily passed around. I expect the requirements for other parts of the Fediverse are similar, but haven't looked in detail.