"Presumably..."
Yeah, don't let your ignorance get in the way there. I'd say it's because tons of useful web apps that don't require running on Windows are a damn sight easier to install and administer on 'nix. For starters there's no need to track licensing and much cheaper to scale, so it's almost a no-brainer.
I don't know of a sysadmin in the land who finds running cross-platform apps on Windows except for a few who haven't tried anything else.
Plus there's loads of choice for the underlying OS depending on your requirements. Most of the are much smaller than a Windows instance alone too, even Core.