I think Ebbie pegged one big part of it: "There was a vitality and drive to his songs that was very different from the Bing Crosby/Frank Sinatra era". The appearance of Elvis may have been the greatest moment of "and now for something completely different" in the history of music, (So far as nation-wide, mass market, awareness of music like that). It may also have been the first time American parents were that widely and fervently galvanized in moral outrage against what their kids were listening to; a lot of that was, as also mentioned above, "it was race music." I was indifferent to Elvis for many years, then heard some of the earliest stuff again, and all I once I thought, "What raw, vital, FUN stuff." I haven't put him at the top of my pantheon, but I've moved him up quite a ways.