It was just a style. The thing one has to remember is the 1970s, the entire decade was the peak of music, movie and TV usages.
For instance in terms of absolute raw numbers “Laverne & Shirley” at the end of the decade pulled in more people and a bigger share than any other program and no program has since come close to it. And they won’t. Why? Because cable came in and fractured.
In the 50s no one came close to matching “I Love Lucy” in terms of a percentage of the total viewers. Part of this reason is there were so few TV sets and TV stations in existance. In deed many parts of the country had only one or two TV stations. Conversely the total number of TV viewers is highest now, but with cable and the hundreds of channels it means no ONE particular show can pull in the same percentage.
“Laverne & Shirley” happened at a time just before cable exploded.
Soft music was just like this. In the late 70s, the number of singles sold and radio stations playing music hit its peak.
While it seems like soft music ruled in the 70s, it really didn’t anymore than disco did in the late 70s. Oh yeah it was the dominant music, but even at its height you still had artists like Glen Campbell (country) breaking into the #1 spot.
All through the 70s if you look at the Hot 100 chart you constantly see soft rock, rock, R&B, country all mixing to achieve the top positions.
Now you don’t see that, because when the Rock era ended it was taken over by R&B, which is mainstream pop music. Indeed by 2005 a new chart the “pop 100” was put into place because the “Hot 100” had become a near duplicate of the “R&B Chart.”
Mainstream music today has aligned it itself to R&B to the exclusion of others. In 2009 Billboard adjusted the “Hot 100” and discontinued the “Pop 100” when it was determained it was digital downloads that were causing the problem. Black people didn’t download music, they bought it, while white people weren’t buying it but downloading it, in a nutshell.
During the history of the Rock era which lasted from 1955 - 1999(?? -roughly) there have always been periods of local dominance that seemed to dominate. For instance “Oz-Rock” (Air Supply, Olivia Newton-John, Little River Band, Men At Work) in the early 80s, followed by the second British invasion (Sheena Easton, Culture Club,the Police, Wham!)
As you see those particular acts just stood out.
Very few acts in the 70s were able to change their style. Olivia Newton-John, Diana Ross, the Bee Gees, Paul McCartney were among those who could change from slow ballads to disco.
Others such as Helen Reddy, Donna Summer stalled. Even Elton John who totally dominated the charts, fell out of favor after 1976’s “Don’t Go Breaking My Heart,” and though he had a few hits, it wasn’t till 1984 with the second British invasion he came back big time with “I Guess That’s Whey The Call It The Blues.”
Another feature is the lack of singles then. Many bands just didn’t put many of them out and didn’t care. Led Zepplin, the Who for example all put out many top albums in the 70s with no real single success.