Sounds very much like no one wants to take responsibility for the land, the owners don't want the job of clearing it so it can be sold, or have to sell it at a knock down price because of it.
Same with the developers, they don't want the responsibility for the same reasons, yet whoever does finally take it on they would surely clear the land, after all no one wanted the old carbonisation plant site at Wingerworth at one time and look how polluted that land was, now there's a housing estate on there now.