I think that when it comes to the arts, we respond most approvingly to recognizable variations on a theme, albeit with at least some distinctive or novel element.
The basic pop/rock/R&B song structure is pretty simple, typically verse-chorus-[verse-chorus]-bridge-verse-chorus, although you can produce songs that illustrate every conceivable variation of the above. Likewise, the usual instrumentation of pop music is generally formulaic: guitar, bass, drums, and optional keyboards. It’s so formulaic, that the relatively few bands that actually have trumpets, violas, saxophones, vintage Mellotrons, toy Casio instruments, or Tesla coils, stand out from the crowd. (And in the case of Man Or Astro-Man?'s Tesla coil stage gimmick, make that, “really stand out from the crowd”.) Bands these days often pair up the bass with as many as three guitarists, but that’s still a variant of the basic instrumentation. (King Crimson’s recent “double-trio” lineup of 2 gtr., 2 bass, and 2 percussion is such a freakish anomaly, I can’t think of another band to have had one, but maybe that’s just my ignorance.)
Given that on the face of it, pop music and band lineups are both so predictable, why is pop music also so delightful to so many? Well, as they say, the devil is in the details. Take the simplest, stupidest keyboard part imaginable: “plink-plink-plink-plink-plink-plink-plink-plink” eighth-notes, one note per bar, two notes so repeated, alternating for four bars at a time, before breaking off for a more fluid cycling motif – a part only a mother could love, and a Rhesus monkey would find challenging to play. Got that? But if it’s played on the endearing toylike Farsifa organ, in a high register, and underscores both the earnestness and goofiness of the lyrics of a song called “96 Tears,” then it’s an instantly recognizable classic one-hit wonder. And it ain’t the lyrics that are that song’s trademark appeal, either; it’s that damn plinky-plinky dunderheaded organ part – and in particular, that four-bar redundant bit! 
That pop music requires a degree of novelty is probably why the truest test of a pop song is the bridge section, and why, when you’re unsure of whether a song really works for you, you wait until you hear the bridge before pronouncing final judgement. Songs may vary in their stanza and chorus lengths, order, and repetitions any which way, but the bridge is almost always eight bars of 4/4. Don’t ask my why; it just is, 99 times out of 100. (One notable exception: Talking Heads’ “Crosseyed and Painless,” which is 12 bars of 4/4). But perhaps that’s why it’s the litmus test for liking a song, and a key showcase for the musicians to demonstrate their skills and put on a mini-show-within-the-show. Just as a good pop song has something to distinguish it from the pack, a good bridge – with its chord change(s), novel instrumentation, instrumental solo, or what have you – is distinguished from the rest of the song; a variation on a variation. There’s thousands of songs that, regardless of how marvellous they are up to that point, really succeed or fail by dint of their bridge section.
Does my basic argument hold up when considering the most influential band of all time? I think it does, although it’s rather like a snake swallowing a baby elephant. What really differentiated The Beatles from everyone else? If you listen to various R&B artists from the late '50’s-early '60’s or to a Merseybeat compilation from the early-mid '60’s, you might objectively conclude that the margin of difference was a conjunction of discrete improvements or novelties, some small, some large, over the rest of the pack: the groundbreakingly fluid melodicism of Paul’s basslines; Lennon’s (& Macca’s, too) unique lyricism, by turns trite, witty, poignant, or surreal; George’s wonderful guitar playing; their marketable good looks, clean image, and youth appeal; their wonderful chemistry as a group; or even Ringo’s steady drumming, which might’ve been just a bit more steady than most drummers.
Yet in their own time they were also dismissed for their teen appeal, their image, their silly songs, the derivative nature of their early output (ripping off black American R&B artists their specialty), and so forth. Only in retrospect does their unparalleled success seem inevitable.