I always assumed there was a lot of high voices in rock for practical reasons. It cuts through the mix better and the girls can sing along to it. Plus, a low voice just doesn't give the required energy in a rockin song. You have to push it to make it fit with the song, and that lends itself to higher pitches. I go back and forth with this, as a singer. I can do the Cash/Waylon thing, but it's like using a totally different muscle than trying to do higher belting-it-out stuff. I always admired Elvis' ability to go back and forth, all while oozing vibrato the whole time.