Very simply because if Jazz hit Sonic once before he went hyper, he'd have to collect another fifty (I believe) rings to even contemplate trying again. In this time Jazz would beable to hold him at bay long enough for the rings to disappear.

Voila. Toasted hedgehog.

Oh yeah, and btw, I believe it's "Super Sonic," not "Hyper Sonic," that Sonic transforms into. If I remember correctly, Hyper Sonic doesn't really even exist in all but one level, and even then, it's a glitch.