I still see young men, street/hood dudes, and washed-up old heads tryna act like they're young sagging; however, I do believe the popularity has waned nowadays. I'm 29, and I haven't sagged since high school, which for me was about 12 years ago and the only reason why I was doing it at the time was to fit in with the popular kids I'd hang around but also impress the girls.
One of the reasons why people, especially young, would sag their jeans, etc, is because it was popular in school, you'd be looked at as one of the cool kids, but also, as crazy as it sounds, it used to impress certain girls in class or around the way because they'd think you were a bad boy/street dude. Also, some dudes wanted to look tough, and sagging gave you some “cred” around the kids who'd hang in the street.
A lot of women nowadays prefer men who wear their jeans and pants on their asses; plus, sagging these days lost that coolness I mentioned and has become associated with bummy behavior. These past few years, seeing somebody sagging their jeans, sweats, joggers, etc, gives off bummy vibes.