Microsoft sold ARM-powered versions of the Surface for a while (not sure if it was ARM64 at that point) and releases a version of Windows which runs on the Pi, so it's something they've showed interest in for a while. I'm guessing that they also see the writing on the wall in terms of ascendancy of the ARM platform and want to make sure they have software to sell which runs on it as it happens, but hardware vendors haven't been falling over themselves to bring ARM Windows devices to market just yet.