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.