I will diverge with the other answers so far and say that he died at the right time.

That’s not to say that he was not a giant in his heydays. But he hadn’t aged well and the times had changed so for most young people he was just that square fat dude in tacky jumsuits singing the old style rock-n-roll music their much older siblings or worse their parents had listened to.

And then something momentous occured, in 1973 an young movie director who would later gain some fame for sci-fi movies released a film called American Graffiti about a group of 1962 high school graduate, on the cusp of going to college or getting jobs, cruising in cars on the streets of a small California town, listening to that old style rock-n-roll music.

The movie was a success and spawned a number of follow ups. The TV series Happy Days, set in the same era and starring one of the stars of American Graffiti, and even Grease which was was released less than a year after Elvis’s death.

Those fuelled a new era of popularity for old rock-n-roll music so that his death in 1977 coincided with the crest of that retro wave and so moved many more people than if it had occured a few years earlier or later.

Also the fact that Elvis was associated with Las Vegas contributed the impersonator phenomenon, those impersonators filled the void and became a fixture there whereas there are many Beatles and other acts cover bands around they don’t have a haven to congregate to like Elvis impersonators do.