Staffies are the victim of a viscous circle. There are too many of them. None are bred by reputable breeders.
Because there are more of them than other breeds, it stands to reason that there will be more of them in rescue centers, that's just straight froward maths.
Because they are not bred by reputable breeders, they are sold to anyone who is willing to pay. Unfortunately the very undeserved bad press that they get and their muscular stature attract the wrong kind of owner. These owners want a 'hard' a dog and soon realise that what they have is in fact a soft as mud, four stone lap dog and more likely to slobber all over the rival drug dealer, than he is to savage him. They get bored and dump them or sell them on, untrained, to yet another unsuitable home. This culture means that the number of staffies in rescues grows out of proportion.
Finally because there are so many of them in rescues and because of the press coverage, they are often overlooked by people who would be ideal staffy owners, meaning again, their numbers in rescue grow.
Then people assume - Too many staffies in rescues = there must be something wrong with them 
Staffies are lovely dogs and do not deserve the image now have.