I disagree. Most serious game development *is* done in C++. Even on iOS (where ObjC is mainly used to interact with the OS ). I also disagree that C++ is not the "recommended language of development" - Neither Apple nor Google discourage it (though you could argue that they initially did) and seem to have pretty much accepted that its not going away for game dev.
Even with liberal use of ObjC in an iphone game, a port to Android is still achievable with a modest amount of effort (methodically going through the code and replacing all the ObjC with plain C++). A WP port requires almost a full rewrite, and maintaining a new version of your codebase. Given the low install base, that's almost always just not worth the effort.
I do agree (based on my own experience) that iOS devs do have a tendency to rely on propriety iOS APIs (yes, ObjC) in cases where there are plenty of open source C/C++ alternatives available (random example - loading .png files). If i was to put my conspiracy theory hat on, i might even suggest that Apple is providing a lot of "useful" functionality through its own (incompatible) apis in attempt to make ports more difficult. With a little care though, a cross platform game that can run on Win32, Mac, iOS, Android, and Linux, requires very little in the way of platform-specific changes. WP on the other hand is just, well... incompatible.
The developers who might be won over to WP over are those who already doing XNA stuff for Xbox live, and/or pc, but this is a limited group. Seasoned game programmers(Console/PC) have spent years programming in C/C++ and are unlikely to want to make the switch. To me (As one of those seasoned devs) even Blackberry's new QNX based platform is way more attractive than WP. I don't know *any* developers who are doing WP ports who weren't paid by MS to do them (And even then, they subcontract the work out)
With regard to you comment about porting android java apps, i think this is a more complex issue than you suggest. Apps that use the Android Java apis extensively are actually much more tied to Android (UI, OS) than those that are built in C++ (its pretty much unavoidable if you do anything remotely useful with UI). To port them you'd need to implement a good chunk of Android on windows phone. Even then you'd end up with an ugly android UI, looking very out of place as part of your WP experience (and given that how pretty it is, is their #1 (only?) selling point, I'm pretty sure MS doesn't want this!).
In summary, I think MS have made a huge mistake - what they've done *might* have made sense if they were a market leader (like iOS), but with their current tiny market share, having such huge barriers for development is almost suicidal.