If you’re asking about adult men, it’s because so many of them have bellies, so they get their pants to fit under the belly. For young men, especially young men of color, it started in prisons and juvenile detention facilities. They didn’t have enough small-sized pants for the increasingly younger inmates so the kids’ pants were way too big. As often happens, this prison reality turned into a fashion choice because it gives perceived street “cred” and/or coolness factor to the wearer.