I AGREE 100% WITH YOU....AN EXCELLENT QUESTION WITHOUT AN EASY ANSWER!!!
For me, the reason Elvis remains a phenomenon today is because he is a clutural icon!! He INVENTED rock n' roll (although some may try to argue this point)...most artists today, when asked, cite Elvis as a major influence!! What would the music of today be like, if it weren't for Elvis Presley?
Elvis had charisma....he exuded confidence and raw energy....and the fans back in the 50's had never experienced ANYONE like Elvis....it's almost as if Elvis tapped into their deepest, darkest desires with his rebellious style of music.
But, perhaps the simplest answer (if there is a simple answer to this question) is
THE VOICE!!! There has never been, nor will there ever be, anyone that could sing like Elvis. And the biggest part of that is, no matter what the style of music, whether it be rock n' roll, blues, country, or gospel, Elvis could sing it all!!! I don't know of another artist in the history of music that could sing ANY style of music, and make it their own. Elvis was able to wrap his voice around the lyrics of any song, in any genre of music, and make it an ELVIS song! (h)
That's the best answer I can come up with!!!
TCB!
Mike