The popularity of leggings may because it could cover your legs if your legs line may not be so perfect, and the dark color leggings could make your legs looks slimer too.
Right now there are many different leggings, in different cloth materials, different colors or camouflage, but all with the elastic power. To see leggings so popular, but there was a saying that the first appearance of them, was due to a famous star who forgot to wear the pants on, so appeared in public just in skin-tight inner pants, and others thought it's the new fashion, and quickly developed the products of this style by big brands.
The leggings make me recall the so called "gym pants" which was popular when I was a child. The pants were also tailored by elastic power, and skin-tight. The cloth material for it is usually very thin. And because of the elasticity, people could also wear that for sports. Just did a search and found that this kind of pants still exist, just not so hot as they were used to. Different from leggings, this kind of pants look more like zentai suits, and there are more colors for selection.
|Fuchsia Tight Gym Pants|
|Dark Blue Gym Pants|