Why is it, that the USAAF never assigned the P-63 Kingcobra the role of low level tankbuster & CAS?

Utilizing its 37mm M4 cannon, four 12.7mm M2 machine guns and a 1,500 lb. bomb load on wing & fuselage hardpoints, the P-63 Kingcobra would've been a formidable Close Air Support tankbuster. At the very least, it would've freed up P-51 Mustangs to focus solely on long-range bomber escort & air supremacy.

JD