SE, Samsung, Motorola, HTC are all heavily invested in Android and are miles ahead of Nokia. They can chose to run an outside lane in that race, or they can lead up Windows Phone 7. All people with WP7 devices are just token efforts, Dell, LG aren't major players like the aforementioned companies are, HTC owe everything they have to Android. Nokia can take WP7 and become the market leader, with Microsoft investing heavily and Nokia providing world class hardware they can provide a point of differentiation as opposed to becoming one of the crowd.

Android is growing, but it's hard for carriers to differentiate themselves on top of it. The carrier UIs (other than Sense, perhaps) tend to be dislike versus stock Android, so that makes the only way to stand out on Android seem less attractive.