Because buying it back is expensive and rescues the capital of those who bought it and asset stripped it (by giving them money for the assets they allowed to deteriorate)

Allow it to fail, let it go into administration then they can legally seize the assets at no cost to the taxpayer under the law that privatised it in the first place.

Won't happen of course, despite the route being clear and without legal issue our corrupt govt would prefer to pay our taxes to overseas interests in the expectation of cushy jobs when they're no longer MPs