Probably because Servo isn't anywhere close to ready yet, unfortunately. It's really cool tech, and I'm dying to use it.
I have a gut feeling that by the time servo is ready, Firefox will have died. That, or servo will never be ready.
Servo may not ever be ready by design, as it is a research project and not meant to stand on its own. The way it's useful is that parts of it occasionally get integrated into regular Gecko as they mature.
The plan for Servo is to integrate it in Gecko piece by piece. In this sense GeckoView (which uses Gecko) will use Servo eventually. It already has WebRender and Stylo from Servo (like the rest of Gecko-powered products).