So far I see a HUGE majority of hosts provide only Linux shared hosting, providing Windows only to VPS (or even to only dedicated servers). Why is it so? While Windows is a lot more expensive than Linux (though it depends on a lot of factors, not just initial and support license cost), it also provides ASP.NET, IIS and of course, Microsoft SQL Server. I know in the past it might have been because of cPanel being Linux only but now they have a Windows version.
But still, why is Linux predominantly used on shared hosting? PHP works on both systems. IIS can be (and probably is) faster. MySQL runs on both systems as well. cPanel has a Windows version. Python, Perl, Ruby, all run on Windows as well. You even have MS SQL Server Express, which I find more superior than MySQL in both speed and features. Access is there for low usage requirements, as is SQLite (which is so great for quick small stuff). And with PowerShell you have a good alternative to the Unix shell.
EDIT: I am looking for common reasons, I realize each hosting company (and/or it's clients) may have different needs. This becomes very important when you get to VPS or Cloud which give you a full operating system to use.
Best Answer
Having worked within hosting companies that provide both Windows and Linux shared hosting platforms it is a combination of cost analysis. As mentioned the licensing costs on Windows is expensive when compared to Linux. As well the Windows servers to provide a stable hosting platform require much more resources than to run a similarly stable Linux platform. Usually required about 4x as much RAM alone to provide the same levels. Even with running Windows "core" there are limits to some of the features that can be hosted so you still can't get away with running just "core" if you want things like .NET and ASP. Another big factor is the fact that you don't have to reboot your Linux servers every "patch Tuesday" as you do with Windows.
In the end it is generally cheaper and more cost effective to run Linux hosting servers than Windows and that makes a big difference. As mentioned the biggest draw for Windows hosting are those needing to interoperate with other MS products and those that require that are willing to pay larger account fees to have that. If you look at most hosting providers you'll find they offer some form of webmail or simple IMAP/POP for email instead of Microsoft Exchange, this is also due to the licensing costs Microsoft racks up if you use it.