Why would foster parents be denied to adopt their foster children?

In the case of our STAKs, their foster parents were not a good fit. The kids were not doing well in their home. At times they wanted to keep the kids but not actually adopt them. They also complained a lot about how difficult the kids were and wanting to swap them for "easy" kids. The kids behavior was not very good. FM did not interact with them well. The sw kept the kids in their home for 2 years, because they did not realize it would be that long, and they did not want to move the kids around too much. They actually came for a long visit to get out of there and then when they went back, they went to a new foster home until they could come back to us. I think if the kids are doing well, you don't have to worry.