Same reason Spez is still doing damage control over 3% of his userbase leaving, up to and including signing up more bots to boost traffic and denounce the protests. It's not like he's losing a whole lot, he's kept multimillions of users (human and otherwise) to make his shareholders happy and his site will stumble along pretty ok for years. But in capitalism, especially the ad-driven digital sphere, eyes are everything.
Federating with the rest of us IS a blip in the grand scheme, but so was 3%. The fediverse existing outside of Threads carries the very significant risk that their users will sign up for their service and see the other platforms that are just as nice without being ad-soaked, subscription-based, and demanding your real name while they sell every ounce of your info.
One of us is going to be leeching users from the other and meta will not like this, so they're liable to make damn sure it's them. It wouldn't really shock me to see them neglect to upkeep federation with non-meta instances while they attempt to charm users away with a high-traffic platform full of bells and whistles.
This is also ground that hasn't been broken by a big name company in decades, meaning whoever gets here first probably gets the bulk of the new users. Other companies could and may do copycat sites, but they don't tend to survive well.