I'm sure that as soon as Windows Phone increases it's market share to above 40% (where both Android and iOS are at) then Stack Exchange will waste no time in developing an official app for it.

As it stands, though, Windows phone doesn't even have 4% market share in the smartphone market, so there's little reason to pursue it.

http://wmpoweruser.com/windows-phone-hits-its-highest-us-market-share-in-2-years-with-more-than-6-million-users/

Consider bringing this topic up again once Windows phone nears or surpasses either iOS or Android.