It was commercial music. And they could and did sing in tune and in good close harmony. As has been pointed out, sharing a mike can help in close harmony. And audiences like to see it.

Some writers who were just getting started probably appreciated the boost PPM gave them: e.g.   Gordon Lightfoot--"For Lovin' Me".

Some of the music still holds up. Most probably doesn't--especially the many saccharine offerings.

But kids probably still like some of it.

In the Folklore Society of Greater Washington, we consciously avoid anything they sang, by and large.   Not that they didn't do it well--it's just that because it was commercially successful, it doesn't need our help to survive. And a lot of music which deserves further exposure can benefit by our attention.