I grew up with the emergence of popular, or pop music in the 1960s. It was part of the resurgence of AM radio, which had been losing market share to FM radio and television for over a decade. AM broadcasters responded by creating niche stations that were meant to provide specific content, like discussions or sports, instead of the entire spectrum of content they had been known for.
These pop music stations that surfaced developed a business format around the length of songs which people listened to in their homes. This long-time format is referred to as ABABCB: A referring to verse, B to chorus, and C to bridge.
The length of popular songs they played reflected the types of vinyl records people played at home. Single song records were initially designed to play at 78 revolutions per minute, or RPM, and were sold on seven inch diameter vinyl records that could have one ~4 minute song on each side. When playback speeds were reduced to 45 RPM, the content and physical size remained the same because of the existing manufacturing equipment.
So, why all the techie stuff? Because musicians wishing to get their songs played were forced by record companies to adhere to ABABCB and ~3:30 duration. No exceptions. Well, no exceptions unless you had the popularity or uniqueness to get past what we had been trained to hear.
Let me cite three examples of three songs that broke the rules, became classics, and opened the door to the longer, more elaborate songs of the 1970s.
(Note: The Beatles had become very playful at this time. The first minute of their video is joking around.)
My first citation is Hey Jude by the Beatles. When released in 1968, they were the kingpins of popular music. It was the first release on their newly formed, wholly owned record company, Apple Records. Stations had no choice but to play it, and adjust their standard programming schedules accordingly. I mean, it was the friggin' Beatles! However, the band gave stations the concession of making it 7:11 long, and thus two song slots. Many have said the lengthy na-na-na--na ending could have easily been half as long and still worked.
My second citation is Queen's Bohemian Rhapsody. Again, it was their extreme popularity that helped pull this off, but that popularity also came from their immense creativity. Queen's musical arrangements had been quirky from the start. Critics viewed the 1975 song as bombastic. Audiences adored it, and the film Wayne's World reintroduced it to a new generation. It is a classic sing along tune that covers a range of emotions, closing with the serene recognition that "nothing really matters, nothing really matters, to me".
My final citation is American Pie by Don McLean. Like Bohemian Rhapsody, it started and ended slow and peaceful, expressed many emotions, and was a blast to sing. Radio volumes were dialed up when the subdued intro was heard, queueing up the listeners for 8:42 of playful release when the the United States was struggling with the Vietnam War and the civil rights movement.
There you have it. All three of these artists were popular on AM radio. Their ground-breaking songs paved the way for lengthier, more creative recordings, and record executives adapted for business reasons. FM stations had already been allowing longer songs, and AM didn't want to keep losing market share. Bands were given license to expand their creativity, and the concerts of the 1970s evolved into lengthly extravaganzas of who could play the longest. It was a win-win for music fans.
(All video links lead to YouTube)