It's because the type of "hey take this number and route it to this network" done with magnetic stripe can't (easily) be done with chip transactions (though it is done with the US Debit Common AIDs).

I'm surprised Discover doesn't just team up to get D-PAS on Chinese terminals. UnionPay will support US merchant terminals, but I don't know if that will just be for those that partner with UnionPay or if it'll be everyone.