Why spend those resources when you have a perfectly good tool available for porting, one that users won’t need to be trained to use?
I don’t buy this. Microsoft have invested huge resources into Windows 8, and have made huge changes to their fundamental values with Metro and Windows Phone (in restricting hardware and app installation etc, as per Apple). So why not go the whole way and develop a File Manager for Metro? I can’t believe they’d get 90% of the way towards a bright future (Metro) only to give-up and decide to throw-in the Desktop as well – the switching between two very different UI’s will be hugely jarring for many users. There must be more to it than that. If not, it seems a very strange decision.