Some providers offer both Linux and Windows VPS, yet they don't have staff that is equally familiar with both.
Beyond the issues of getting support for the different operating systems, there is a security factor.

Everyone knows that Windows Servers are the most vulnerable, even when hardened compared to Linux.  Ask anyone Administrator responsible for both Linux and Windows servers, and they'll always be the most worried about Windows.

Since we don't offer Windows servers, our network is much more reliable, and secure compared to VPS Providers offering Windows Servers.  Even if you order a Linux server from a provider which offers Windows Servers, you can still be at risk.

Imagine if the VPS Provider has several VPS or customers with unsecured applications, that get exploited.  They could be used for DOS attacks or other malicious attacks,  which will result in less network stability, possible downtime and at the very least, high latency and slow responding web pages and e-mail servers.

We always recommend that people looking for a Linux VPS, choose a company that deals exclusively and specializes in Linux VPS.

At compevo, we've decided to ensure all of our servers run Linux, including customer provided Clustered Linux VPS's.