A license for Window Phone isn't much more then hardware manufactures already pay for a Microsoft Android license not to mention the cost of patent litigation costs to defend their use of Android. Going to Windows would be cheaper. Differentiating themselves from other Windows Phone licensees would be a problem, but that is still a problem for Android users. They don't advertise the customizations to Android, they just advertise their phones as being Android based. Moreover, the varying flavors of Android really aren't that different.
Further, most Android phones are cheaply made, even Samsung's top of the line Galaxy phones. The materials are cheap plastic. Nokia actually makes decent hardware, and if I weren't an iPhone user, I would go with Nokia.