No matter who the artist is you have to put things in perspective with regard to the era they burst on the scene. In a time where most performers were plain and straight laced. Elvis cut a completely different image from his peers. He had "swagger", his hairstyle was unique, and he could dance. In fact some TV shows refused to show his hips moving while he performed. Girls were crazy about him and this led many guys to want to imitate him. Whenever an artists becomes an icon or brand due to their looks, clothes, speech pattern, outlandish behavior and the toys they own (private jet)..etc
Today much of this is commonplace but during the 50s & 60s he was a trailblazer with hit records, sold out concerts, and hit movies.
Last but not least anytime someone is crowned "The King" of anything there is usually a correlation with their record sales, and the amount of time spent at #1 on the charts. As I said it's all relative to the era the artist arrived.
Prior to Elvis there was Frank Sinatra and after Elvis came The Beatles with record breaking sales and concerts, and then came Michael Jackson where some estimates claim "Thriller" sold over 100 Million copies worldwide. Very few artists become icons.
Nevertheless I'm sure pre-teens of today will wonder what was the big deal. In order to fully appreciate these artists you have to have been around to see their explosion on the scene versus their peers.