I know you're looking at this from the handset maker's point of view but I think it's equally applicable to the consumer. Exactly why should I chose a Windows Phone over an iPhone or Android phone? At this point, you can get Android phones and iPhones for free with a subsidy, so they can't compete on cost.

Windows Phone may be nice, hell it may be better than Android, but there is no "OMG that is so cool I have to have it" feature. Until Microsoft can find a way be not only better but different, they won't regain marketshare.