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

Thanks for all of the answers. I guess this seems to be more common in states that do not allow foster/adopt dual licensing. It makes sense when it is age or health related or if it is a "bad" match. When I was hearing it all I could picture was this young couple who had fostered their kiddos for a year or more and given them the world only to be told they would not be allowed to adopt.