Not an expert, but I'm currently taking a History of Rock class and I might be able to shed some light on this, regarding Elvis specifically. First a little bit of background:

It's generally agreed upon that the common ancestor of almost all popular music of the 20th century began with ragtime in the early 1900s – jaunty, upbeat piano music that you'll often hear when watching old silent movies, and Scott Joplin came to be regarded as the King of Ragtime. Going from here, we get jazz – which more or less diverged into its own distinct genre around the time of the beat generation – and we also get the blues, which has perhaps been the most dynamic genre of music in America. We start with classic blues (e.g. Bessie Smith), then rural/delta blues (Robert Johnson), gospel (Soul Stirrers), Chicago blues (Muddy Waters), jump blues (Louis Jordan), club blues (Nat King Cole), boogie-woogie (Pinetop Smith), doo-wop (The Ink Spots), and pretty much all of this blends and culminates into rhythm and blues, which at this time is extremely close to what actually came to be known as rock n roll.

Looking at all this, one should note that literally every person I've listed so far was black. Up until the time of Elvis, almost every significant advance in western popular music in the 20th century (thanks /u/Dak105) was done by black artists, but large record companies were not keen on releasing so-called "race records" for fear of alienating their white customers. A solution to this was to produce sanitized, dumbed down covers of popular black music (Pat Boone) that ended up topping the charts, despite being vastly inferior to their source material. One of the first big breakaways from this was Bill Haley & His Comets, whose cover of "Rock Around the Clock" was, for lack of a better term, actually good, and was extremely successful.

Soon after, Elvis comes onto the scene and becomes wildly famous by covering songs originally produced by black artists. But unlike Pat Boone, Elvis' marketability was not based on being a safe, wholesome artist that white parents could enjoy along with their kids, catching a lot of flak for his dancing style on stage. Growing up in the south, he was heavily influenced by the music of his black neighbors, and melded this with elements of country and western in a style that came to be known as "rockabilly." So while it wouldn't be an exaggeration to say that Elvis stood on the shoulders of black artists and popularized a segregated genre of music, his versions of songs have significant stylistic deviations from their original recordings, while at the same time not watering them down. There's also the idea that the singer is more important than the songs, and Elvis was probably the biggest example of that notion up to that point, being the legend that he was. This requires a somewhat looser interpretation of rock n roll to encompass things other than just the music itself, but hopefully this all helps to answer your question.

edit: clarification

The most important thing to consider when talking about Elvis and Jerry Lee Lewis was the white appropriation of black culture. Extremely interesting stuff.