Because it's more fitted then sweats. It's an alternative to sweats. It brings out the shape of a girl rather than making them look like they have no shape in sweats, aka the butt. It's casual but in a more classy way. If you are looking for some, lululemon makes the best yoga pants, but they are quite pricey. But they give you such a great shape and makes you look super sexy. They are super popular where I live.