Linux, Windows, "Marketshare"
It might be true that the sales volumen of Windows server hardware is larger than the revenue for Linux servers.
But the large workloads in Stock exchanges (Frankfurt, Tokio,..) , Google, Facebook, large SAP installations, large file servers, large websites - I am sure they are first and foremost running on Linux and Unix.
The underlying reason is that the Linux kernel is much better suited for scalability, efficiency and I/O throughput. Also, operating a system by a proper Command Line is much more efficient than a point-and-click-based system such as "Windows" (for the professionals, of course).
Another case of "lies, damned lies and statistics", I guess.