Staffs end up in rescue in such high numbers for many reasons, the main ne being there are just so many of them. They are bred in their thousands by BYB every year. These breeders don't care if the homes they sell their dogs are suitable. While Staffords are great dogs, Stafford puppies are not for the faint hearted or unexperienced. Adult staffs are brilliant for first time dog owners, onve they have finished the puppy stage.
Another reason would be their appearence attracts the wrong kind of owner. They are bought by muppets who want an aggressive dog. It is extremely difficult to train a Stafford to be aggressive towards people, they are far too loving. Once said muppet realises he has a 4 stone lap dog, he dumps it.
Thirdly because there are so many of them in rescues nice families presume there must be something wrong with the breed and over look them in favour of the more 'typical' family dogs like the labs.
Add all this to the daily mail hysteria surrounding Staffords and they don't stand a chance, poor things. It's such a shame because they truely are wonderfull family dogs, far more suited to life with young children than most other breeds. And they can actually smile.
Dogs in rescue that need a resident dog are ex breeding stock who have never been without the company of another dog and usually have never been in a home environment either 
. They are often locked in tiny sheds and cages, forced to live in their own excrement. This is why regular posters in the dog house get so pissed off when people claim they simply must buy a puppy and can't wait/can't be bothered to find a reputable breeder. Puppy farms won't show you the sheds. They pose as BYBs and regular families, when in truth you have no idea where the freecycle puppy lived for the first 8 weeks of it's life or what it's mother is going through.