Well, it was the 'style', but also celebrities like First
Lady Jackie Kennedy Onassis frequently wore large round and/or square-framed glasses in the 60s, nicknamed Onassis glasses, whose popularity extended into the 1970s,.
The rise of paparazzi culture began to sweep the world during this time,
so oversized glasses were very popular for celebrities to try and hide from the paparazzi (& to protect their eyes from the photo flash).
The 'Jackie O' style was all pure class! She is shown here with a Long Sleeve tucked in collared foulard print blouse, smooth front tailored trousers, black leather loafers, signature sunglasses, long flip bobbed hairstyle, large black handbag with silver grommets and long black pendant necklace: