I think that sometimes the foster carers are inexperienced. I once lived in a regional town where a small group of women started a rescue to try and stem the tide of dogs being euthanaised in the local pounds.
They were dog owners themselves but were deperate for foster homes as this limited the dogs they could rescue, particularly as many experienced dog owners had a full complement of their own dogs. They often just took who they could get.
They mainly rescued the dogs that were not working breeds as they just found that these dogs often needed far more than their foster carers could provide, were difficult to rehome and were often returned due to their propensity to jump and escape.