I think they attract different audiences. Threads is just Instagram with text above, instead of below, pictures. It is more pop-culture oriented, and has all the big celebs and brands (way more than the few dipping toes in Fediverse). It's so far also a more direct Twitter replacement — everyone just signs up in one place, and nobody hears "well actually" about missing replies or non-working follow buttons (I expect it will be a mess if they add federation).