If you're truly interested in this topic, it might surprise you to learn there's no recognized 'pitbull' breed. It encompasses a range of canine subspecies, including but not limited to the American Pitbull Terrier which is likely the particular dog you're referring to. But it's also used to describe any mix of bulldog and terrier, or Staffordshire terrier, or American Bully etc etc so there exists a much larger number of dogs that will be described as a 'pitbull' than say Doberman Pinschers or Rottweilers or Cane Corsos. Hence it's statistically unsurprising that a majority of dog attacks are committed by dogs that can be described as pitbulls.