Because it's the perfect Enterprise OS for companies, schools and other organizations that have moved to the web. Currently there's just no OS that would rival ease of deployment for large scale thin clients.
Android on the other hand has very limited provisioning tools and even iOS/OS X is orders of magnitude more expensive and harder to provision, keep updated and running.
The cost is not just in the form of development expenses. It's about having to support two systems, creating a confusing message for consumers, having to foster and support two different app ecosystems, marketing two different competing platforms, etc.
Android on the other hand has very limited provisioning tools and even iOS/OS X is orders of magnitude more expensive and harder to provision, keep updated and running.