To answer the original question...He was an absolutely incredible live performer when he was young, that's why. Simple. He had the voice, the look, the moves, the charisma...devastating. After the stint in the army he began a long, slow decline, ruined by the fame and the great unholy $y$tem all around him.

Now, if that ain't good enough for ya, duelling-etc, let me come up with another answer.

Hmmm.

Okay, people rave about Elvis because they don't want to admit to secretly harbouring an unquenchable desire to share a hot tub with William Shatner! Yeah, that's it for sure...