Irresponsible owners.
Unfortunately, probably 90% or more of the people who own pit bulls cannot handle them in extreme situations. That includes a large percentage of PB owners who believe they are dog handlers but won't be up to the task when push becomes shove and that centuries-old instinct of attack kicks in. Personally, I think that anyone who would want a pit bull is probably irresponsible in that they greatly overestimate their ability to control the dog.