Color blocking, especially with black caught on big time in cities everywhere. The specifics didn’t really matter, industrial run off, pollution, trash mountains, encroaching ocean waters, there’s always a reason why the ground is soiled.
A lot of that stuff just doesn’t come off. It's possible to avoid in the street, but eventually, everyone is going to sit down somewhere, and most surfaces aren't any cleaner than the ground is. Someone, somewhere must have ruined one too many pairs of pants, and the dirt hiding black look was born.