Good question! I have wondered that as well! ☺
Really it comes down to this, black jeans have been dyed, dyed again and yet died again, just to be black. To make that color stays they have to use a ton of chemicals and the end result they smell like sulfur! If you love the smell of sulfur then you're just weird and you can ignore the steps below! If not then try the "Smelly Jean Washing" recipe below!