Also, the Lumia 920 is the most popular Windows Phone device (source). And it's a high-end device. That shows how well they're doing on the hardware side. And speaking of the hardware side, WP allows them to be unique and not just with the OS. The Lumia range of phones are offered in a variety of colours that go well with the colours of WP. If they went with Android, they'd have the same issues as any company that isn't Samsung (but to a lesser degree because it's Nokia and their hardware is awesome).
Unique? hah, what a joke. It's basically the same end experience with slight variations to the CPU, RAM, Camera, and case. Not to mention the fact that Microsoft limits what they can and cannot design into their handsets. You get none of that limitation with Android, hell pretty much any part of it apart from the Google Experience applications can be customised.