Because it isn't. XNA isn't supported for Windows Store apps, but continues to be supported for Desktop mode.
So like Silverlight then. Quote from your link: There is not a version of XNA Game Studio that works with Visual Studio 2012. You can install Visual Studio 2010 side-by-side with VS 2012 though. - Encouraging!
MS employees promote SharpDX and Monogame:
Sure signs that XNA is alive and kicking.. of course it's "supported" (a.k.a. keep-alive), but as I said, so is Silverlight (well, SL is actually "more equal" than XNA - you can still do SL with VS 2012).
Do you think there will be a XNA 5?