(Answered by W Boutros)
For a variety of reasons, but the short answer is that they had two superbly talented lead singers rather than just one, but much more importantly, both became extraordinarily talented songwriters. John Lennon and Paul McCartney may or may not have become successful on their own. But together they complimented one another’s songwriting talents perfectly, such that they became immeasurably better than they probably would have individually or under different circumstances.
Lennon was more lyrically adventurous, which inspired McCartney to try to match him. Likewise McCartney was more gifted musically, which inspired Lennon to up his game. So in the spirit of friendly competition, Lennon & McCartney spurred each other on and became the most successful songwriting duo in history (by records sold).
That creative fuel in turn inspired George Harrison to excel as a songwriter (who also occasionally sang lead). Harrison’s wonderful vocals also made for impeccable 3-part harmonies with Lennon & McCartney. And Ringo, who also sang lead on a few songs, was the solid backbone they needed to evolve as a cohesive unit. Together they expanded musical boundaries in revolutionary ways, and more quickly than any musical act before or since. They recorded their entire catalogue, including 20 #1 hits (US charts), in only 7 years. That’s a staggering thought when you consider that most modern acts release maybe 3 – 5 albums and perhaps 10 or so singles in that same time-frame. And their hits were just the tip of the iceberg, many people would agree that their albums and album tracks had an even greater impact and lasting influence on pop music.
Put it this way, from the time you first heard of Ed Sheeran (say, 2011) until now – the Beatles would have done what they did, changed the world, and broke up in 2018.
Also, unlike many other acts, the Beatles rarely repeated themselves, which is why listeners didn’t grow tired of them. Beginning roughly around 1965 almost every new release was a radical departure from the prior. They kept pushing boundaries, which is risky, but it paid off. The range of their stylistic diversity is arguably unparalleled for a major pop act.
Another, often overlooked, factor is that they were surrounded by the right people – especially their producer George Martin and manager Brian Epstein. If they hadn’t captured the interest of Epstein (who had never managed a band before) they almost certainly wouldn’t have wound up in the position of being signed to Parlophone by George Martin – who decided to take a chance on them after they were rejected by other labels.
George Martin’s importance cannot be overstated. He was a classically trained musician, yet willing to experiment, so he helped guide the Beatles and turn their often radical ideas into reality. He was crucial to their ever-evolving sound.
Finally they arrived at the right time and place in history to touch a cultural nerve in a way that was virtually unprecedented. Some sociologists theorize that the JFK assassination, which occurred less than 6 months before Beatlemania, created a void or absence of joy (at least in the US) which needed to be filled. In addition to their exciting new sound, their natural charisma and irreverent humor was a refreshing delight to the press and the public. Regardless of the causes, they became a cultural phenomenon that remains unmatched to this day.
It’s not that John, Paul, George and Ringo were the most talented musicians on the planet – it’s the fact that they were in an environment that nurtured and inspired each of them to attain most of their full potential. Or to become self-actualized, to use a psychological term – as individuals, and as a unit.
Many, if not all, human beings have the latent potential to achieve great things, but very few wind up in environments which make that possible. The Beatles were four talented individuals who grew to become extraordinarily talented, and who emerged on the world stage at precisely the right moment. For the Beatles, all the stars happened to align perfectly, and the whole became far greater than the sum of its parts.
Every Moment Has A Song 