Mostly it is due to the pesticides and insecticides, which is a shame:
Cotton uses approximately 25% of the world's insecticides and more than 10% of the pesticides. It takes roughly one-third of a pound of chemicals (pesticides and fertilizers) to grow enough cotton for just one T-shirt. Indeed there are organic cottons available, though hemp is more durable and requires little in the way of water, or insecticides.
It’s sad really because so much of the world population depends on cotton production, but the way in which much of it is manufactured is making the planet sick.
Once again, you can't get high off hemp; it's the stuff that goes into making rope. One of the largest PACs against hemp production I note (before the current laws) was the cotton industry and they perpetuated that myth.
Anywho, I still love my cotton, but was wondering about hemp. I had a shirt someone gave me that was nice but had a pot leaf on it so I wouldn't wear it out (I'm not a counter culture billboard). The material however was beautiful. It felt great and lasted forever.