I think it's just as simple as any other language people get used to. Whatever language most of your programmers or founders are most comfortable with is what most products / websites get written in. If you really like PHP, you'll write in PHP. If you spent years writing Windows apps, you might use ASP / C#
That, and a possible explanation is Windows licensing. Many major companies that use Windows desktops in their offices generally have Windows servers to provide Active Directory, print services, file sharing on the lan, Exchange for email, etc... It becomes a "well, we run Windows servers anyways, so why not run IIS and use ASP"