Why do some countries ban certain dog breeds?

Does Japan allow pit bulls?
According to MCO 10570.1 (31 Jan 13), full or mixed breeds of Pit Bulls, Rottweilers, and canid/wolf hybrids, Doberman Pinschers, and Chows are prohibited aboard Marine Corps Installations. There are no dog breed restrictions for entry into Japan, or for treatment at the U.S. Army Veterinary Treatment Facility on-base.