Today has seen a lot of commemoration of the first appearance of the Beatles on American television. In watching footage from the occasion, one is reminded that the band, particularly in their early days, was in some ways what we would today call a "boy band"—young men whose popularity depends not on the music that they play, but rather on the fact that they are found attractive by hordes of irrational, screaming teen-aged girls.
John, Paul, George, and Ringo were, of course, much more than a boy band. But understanding this aspect of their appeal helps to explain why there will never be, and indeed can never be, another rock band as big as the Beatles. The Beatles came along at a time when rock music had not yet diversified into myriad genres and sub-genres, and so the Beatles could be all things to everyone. They were One Direction, and Metallica, and Sigur Ros, and Vampire Weekend, all at the same time. No band today could be regarded as both a boy band and a serious rock band, let alone simultaneously satisfy the fans of pop rock, hard rock, art rock, blues rock, and all the rest.
(As I wrote this, I noticed that the spell-check function on my computer was flagging the word "Beatles".)