Fashion. It's the new craze to wear leggings, tights and yoga pants out in public. That's just what is in.
Comfort. Jeans can get kind of annoying and skirts and dresses can be a bit dangerous. So they resort to the comfort of the leggings and tights/yoga pants, the material they're made of is more breathable etc..
Attention, they like when men look at them but not stared at. They know we like seeing them in them but that doesn't mean we should just stare, at least have the decency to talk to them.
Cheap. They are relatively inexpensive, especially the non name brand stuff. Jeans can get pretty pricey (even non brand) and so can dresses/skirts..
That about does it
The only other reason why they probably might do it, is basically, to piss you off