Because those guys knew how to tell a story and we all love a good story. They told a story at two levels; firstly via their songs, starting with the simplest expressions of romance in Love Me Do through entire cultural summaries in Penny Lane, alternate realities in I Am The Walrus and back to the essential simplicities of Let it Be and Across the Universe. Every one a cracking good story.
And secondly, their own lives made and makes a story we cannot fail to enjoy. Of Northern English towns. Of four lads done good. Of changing pop music for ever. Of re-invention on a mind-boggling scale.
That's one reason we love The Beatles.