First, The Beatles broke the line between performers and writers though it took until their third British album (A Hard Day's Night) to deliver a set that was 100% original material.
Then, the DJs in America broke The Beatles in the USA. Until that happened, British kids who learned to play guitar during the skiffle boom had nothing much to look forward to. But when The Beatles broke big, it was obvious there were big bucks out there for guitar bands who wrote their own material and owned their publishing. Doing that, they got three bites of the cherry as far as the revenue was concerned.
From 1965, there was a flurry of experimentation in an environment where anything might sell, and a lot of different material got played on the radio, which was still trying to cover most of the bases. One points to the quantum leap from She Loves You to I Am the Walrus, and questions how this could be so. And to find an explanation, we need separate looks at what happened on either side of the Atlantic.
In Britain, the interesting part of the sixties started with survivors of the skiffle era playing various permutations of early rock, skiffle, blues, R&B and jazz. And they played it in and around art schools to an audience that hadn't existed before.
Teenagers, at least in the sense we know them today, are a relatively recent phenomenon. They emerged in the late forties and early fifties on both sides of the pond as the post-war economy kicked into overdrive. It took some people a while to realise literate kids with disposable incomes at unprecedented levels were a market waiting to be exploited. By the mid-sixties, you had Carnaby Street and modernist ('mod') fashions ready and waiting for Sgt Peppers and flower power.
And those skiffle era survivors had a problem with the size of the repertoire. Even if you were drawing from a couple of genres, there was a smallish pool of material to draw on. That is part of the reason the likes of Lennon and Macartney, Pete Townshend and Ray Davies started writing their own.
The Response Across The Pond Folkies and Folk-Rockers East and West Coasters San Francisco