The thing that confuses me about the KF-21 is that they went for caret intakes like the F-22, rather than Diverterless Supersonic Inlets. You don't want DSI's if you want to go above Mach 2 but KF-21 is only specified for up to Mach 1.8 (Makes sense considering it uses licensed Super Hornet engines), so they would have worked fine. They're stealthier and lighter too.
I dunno, it's above my paygrade. Would be interesting to hear the reason though.