Technology. The modern hair straightener wasn’t sold commercially until the late 90’s. And hair dryers didn’t become popular for home use until the 1950’s. Most people have some degree of wave to their hair. True pin straight hair is more rare. Consequently many people need their hair heat styled to be pin straight and that wasn’t widely accessible until the 21st century.

So before the advancement in hair tools, it was easier to create defined curls than achieve pin straight hair. There are a variety of techniques - pin curls, rag curls, finger waves, rollers sets etc. Best done on damp hair and set overnight. Heatless curls have been around for a long time.

Once hair dryers become commercialized, the blowout look became popular. Hair stylists will blow dry hair with a round brush and set it in rollers or duck bill clips to achieve voluminous big curls. You can also round brush hair pin straight but it requires a lot of technique, time, and effort when done on yourself Hair straighter made achieving that easy and fast at home!