Don't forget it was the bike boom! I was 15 at the time, living in Canada and wishing I could ride *my* overweight department store bike (at least it was a Chiorda) on the great American tour. It was covered heavily in various media so not just a 15 minute film. And bikes were being sold by the millions so lots of interest.
On the disk brake - it was indeed heavy and even though we rode entry level bikes we dreamed of (and eventually did get) the lightweight bikes we know and love now. They were expensive too, so no one except an OEM would bother fitting them.