I'm not a lawyer, but presumably because there's US antitrust law, under which the courts may find Apple's contractual impositions to be illegal. They may also find they are legal, but if the court has any doubt they should grant Epic's request to stop retaliation.
I doubt this is antitrust. There's a zillion other places to sell a game (as Epic has shown). If you want to claim the entire market in question to be Apple products, I doubt that will get very far. Everything is a monopoly when you make your market hyper specific.
This isn't some hyper specific market. Mobile games are a $76 billion industry (and rising) with the overwhelming majority of users only having access through either the Google play store or Apple store.
$76B is world market. US law doesn't apply to a significant part of that.
Neither Apple nor Google is a monopoly in mobile games. By revenue Apple has about 60%. Check [1], search threshold to see what cases often require.
Taken together they would be, but then you cannot prosecute Apple and win without showing collusion between them and prosecute them both for collusion.
