Key factors Linux winning over Windows Dedicated Servers
1) Security - Linux provides more secured platform, compared to Windows. Even though both Operating Systems have loop holes. But, when both are compared linux would win with some margin in more security.
2) Costing - This is most familiar factor which everyone knows, licensing costs differs alot with Windows and Linux. Linux available as an Open Source, where as Windows costs monthly.
3) Service Providers - As Linux platform is more popular in Hosting Industry, you will find many service providers available with Linux Hosting. They wont fall short for Linux Administrators, getting the server and OS issue resolved. When comparison to Windows its different case, you will find Windows having constant updates and bugs appearing, even difficult to find a good provider with management available.
4) Stability - As stated in early point, Linux is more stable platform. There are rarest of the cases where Linux OS getting crashed. Gives highest percentage of uptime when compared to Windows, even with large number processes running. Windows stability decreases drastically, when large number processes are running.
5) Hardware - With regular upgrades and Updates of Windows OS, hardware resources consumption increases, there is constant requirement of increasing the hardware configuration. When this is compared to linux is flexible and scalable, can run on any server and configurations.