Quite small (cigarette pack sized) electronic flashes were available in the late 1960s and early 1970s. They were initially fairly costly, but the prices fell dramatically towards the end of that period.
"Hot shoes" were not yet prevalent and the flash unit had a trailing lead which plugged into a socket on the camera (my 1968 Minolta SRT101 has such a socket).
As has been pointed out, many people still had relatively ancient cameras (although I remember new box brownies on sale in the late '50s).
Those with 35mm cameras with adjustable or (OMG!) interchangeable lenses were widely regarded as "geeks", just as people with fully featured DSLRs are regarded today by those whose photography is limited to that facility on their smartphone.
The received wisdom was that the "big money" was in simple cameras, with prettier looks, but very little changed from box brownies. Instamatic cameras did adopt the standard 35mm film, but used a square 24mm
format inside a loading cartridge which did all the messy stuff.
They were successful, and did offer flash contacts and accessory flash guns using bulbs (I had such a setup). Kodak was a "one stop shop"—you bought the film in their proprietary cartridge, and flashbulbs in the traditional yellow packet.
Agfa came up with a similar concept, but never quite caught up. As time went on, people became more affluent as well as dissatisfied with Kodak and
their attitude, so the Instamatic market died away, as much of their market bought new Japaneses SLRs, and joined the once despised geeks.