With cross-platform compatibility, specifically mobile compatibility [on platforms people actually use, i.e., not Windows Phone], being a major sticking point, .NET is becoming less and less desirable on the surface level.
Microsoft is just like Oracle. Big fancy businessmen who know nothing about development automatically buy into the MS stack just because it's a big name and the labor is easy to retain. Hardly any technical considerations go into it, and then they're upset when they [eventually] learn that the end result won't work with the iPad.
What about the alternate explanation that Windows Server is growing because demand for the products that already use Microsoft's stack is growing, despite the stack itself? What about the explanation that people are buying more server licenses than they used to due to increasingly widespread virtualization deployments? Whereas people used to buy relative big metal and run one OS on it, now people are taking that hardware and deploying many small VMs across it, each requiring its own license?
There are plenty of interpretations of that data that aren't "people are flocking to .NET".