With the arrival of the pantyhose in 1968 a means of wearing stockings as underwear first became possible. Prior to this the stockings were always held up by suspenders either from a corset, a girdle or a suspender belt.
I think this was also the time that a new fabric was invented which was stretchy enough to allow for the sewing of the cotton gusset into the stockings crutch. This was essential for women because the nylon against bare skin irritates and causes infections. 
Even today, a woman should never buy or wear stockings that don't have a cotton gusset sewn in. Even if a woman wears panties underneath her pantyhose, she should still always only wear stockings that have the cotton gusset sewn in.
This is because the nylon is not a natural fabric. It is a derivative of petroleum and one of its downfall properties is that it doesn't 'breathe'. What this means is that the air is unable to circulate freely causing heat to build up. Because it is basically a moist environment that it is blocking the air from circulating around, this invites infections like Candida to thrive. Candida is a yeast infection that is naturally occurring in every human body, male and female.
The freedom offered by pantyhose underwear to women everywhere was phenomenal. It was the late 60's and the revolution was well and truly underway so it was a really good time for the release of a new stocking that gave women freedom yet preserved enough of the 'well-dressed-socially-acceptable' expectations of the day to continue.