It made bubbles of, per wikipedia,
polyvinyl acetate dissolved in acetone, with ethyl acetate plastic fortifiers added. The acetone evaporated upon bubble inflation, leaving behind a solidified plastic film.
Which looked like,
↓↓ VOLUME WARNING! LOUD DISTORTED AUDIO!! ↓↓
And here’s a 1970’s commercial. Especially headphone users, would recommend starting low and easing into it.
That might have been the actual reason, not the given reason. Sometimes it’s easier to go with the easier explanation then getting into the thousand why’s game.