Because Master Chief has the voice of an angel. Actually, it's been forever since I've seen one of those late-'80s song collection commercial parodies, complete with barely altered song titles to match the "wacky" singer. I'm assuming the folks behind this are parodying the lameness of the parodies of yore, in which case they did an excellent job and elicited a rueful smile, not just because Master Chief sings his darn heart out. But still, let's no one do this again, at least for another decade or so. Thanks to Mike S. for the tip.