Environment. And if you're cleaning stuff all the time, too much exposure to petrochemicals is a bad thing.
Mineral spirits, kerosene, or diesel fuel work better and aren't so environmentally awful if you re-use them. The gunk settles out in the bottom, and stays there if you don't shake it up too much when you pour it. I've got a jar of mineral spirits I've used for cleaning parts for years. It has about 1/2" of sludge in the bottom. But I clean my chain often and use the bio-degradable stuff. Go figure. :confused: