It's quite simple. There is no longer any great social pressure on girls and women to be girly and womanly because traditional gender roles are weakening and fading. And it's just a fact that jeans, t-shirts, and close haircuts are just much easier and more convenient.
That's a separate thing from why some people go way off the deep end with trying to look rebellious. Gods help me, I have a younger sister who's into the whole goth/scene thing. Leggings covered in inverted crosses, t-shirts that say "**** YOU WORSHIP SATAN", handbags and shirts with pentagrams all over (which I kind of like, because she gets absolutely
pissed when I tell people she's an aspiring graph theorist), several pairs of jeans that I am rather certain were fished out of a dumpster after being sent through a woodchipper, and the whole half-shaved Skrillex hair thing.
As for "non-conformist" subcultures in general, I'll defer to Dr House:
Dr. Gregory House: Nonconformity - right. I can't remember the last time saw a twenty-something kid with a tattoo of an Asian letter on his wrist. You are one wicked free thinker! You want to be a rebel? Stop being cool. Wear a pocket protector like he does, and get a hair cut like the Asian kids that don't leave the library for twenty hour stretches. They're the ones who don't care what you think.