It doesn't really have much to do with the practice of Yoga..... it's just how cultures evolved.
I'm only recently out of school and I can remember back to the 90's. I remember the evolution of clothing styles. It's largely based on the Media's messages to the youth and clothes becoming more and more revealing....
Then someone discovered Yoga Pants.... Tight...Revealing...Fashionable... and not to mention Comfortable.
Not much more to it. Styles change.