In the middle of the 60s, the age of the "Standards" began to die. The earliest form of the "record industry" was sheet music. "Standards". It was the songwriter that was the star because there was no radio to attach the song to a specific person(s). Then Vaudville and other "traveling shows", (or the opera and orchestra) might have created singing/music stars, but until recorded music was created, and radio became a "Thing", we had songwriters and regional/local versions of the same song being sung and that is why there are so many versions of, say somewhere over the rainbow
The days of Big Songwriters writing songs that 50 people would cover to varying degrees of success were beginning to end. The "DIY" of rock and roll began (and the covering/ripping off of blues) The songs were simpler, but more personalized, and more diverse. So from the early 50s to the end of the 60s, we had a lot of singles artists making their own songs (for the kids) as the Singer/songwriter phase (for the adults) went away.
By the 70s, FM started to get momentum, and those stations began to play entire sides of albums, or whole albums. By listening to these sides on the radio we "discovered" songs we wouldn't have normally. The prog rock groups began to copy the original "concept album" that the Beach Boys and Beatles (and some say Sinatra with The Voice) pioneered.
But then again, maybe record companies pressured artists to make "an album's worth" of songs to sell the album. Maybe it's both.
Well Frank Zappa explained what happened.
In the 60s, it was the old guys with the cigars who decided who would get signed, their attitude was “who knows, we’ll give it a shot, maybe it will sell.”
Then in the 70s, the young “hip” guys took over and since they were young and hip, they just “knew” what would sell. And thus enter “Corporate Rock.”