carriers are about money. I bet Microsoft could get them to have someone hang out in the Windows Phone section. I see HP, Epson, Microsoft, etc. reps in BestBuy, especially during the holiday season. Just because something isn't being done now, does not mean that it couldn't be successfully pitched.
From all this, I conclude that WP is not a huge priority with MS, since they do not seem to have that kind of money to spend on it.