In addition:
Blame it on the so-called British Invasion. Thats when hype and appearance began to be just as, or more important, than the music. Also, TV can share the blame. Sort of like talking pictures. When we could hear them, some great actors didn't make the cut. Great talent that didn't look so good, suffered the same fate. The recent Super Bowl half time show is a case in point. Beautiful woman, forgettable music. I use the word 'music' loosely. Did I mention she was beautiful?
Cheers