In the 50s/60s, "Police Dogs" (aka German Shepherd Dogs) had the bad rep, then Dobies got popular with the wrong sorts and became the 'dangerous' breed in the 70s/80s - Rotties came next in the 90s, and it's been Pits ever since.
In my observation, whatever breed gets popular with the 'tough guys' (bikers, gangs, whatever) ends up with the bad rep, mostly due to irresponsible people (breeding for traits other than temperament, mistreatment, training for aggression, etc.)
We've had one or more of all of these types (all but one rescues, all neutered, all great dogs).