Because Threads didn't federate. It turns out when they said that they'll federate, they actually meant "some time in the undisclosed future." And then Threads lost a lot of that initial marketing hype so everyone forgot about it.
Apparently Meta is currently testing federation for Threads, though? The problem about Threads federating isn't resolved, to be entirely clear. It was merely that everyone, Meta included, just decided to kick the can down the road and think about the issue later.