Microsoft's tooling is easier to use and they've got a massive training/certification programme.
Because it's so easy, there are a factor more Microsoft admins around than there are Linux admins. And they're fairly productive.
Plus, Microsoft software works really well together. Windows + Windows Server + SQL Server. Works great.
Linux is great if you have highly skilled technicians to support it. There's way more tweaking involved than with Windows; this makes it ideal for anyone running a centralised model where they're deploying thousands of identical machines (like the web). It's not so great for corporate environments where there's a high level of variance..
It's basically the Ford vs Ferrari argument: Microsoft enables companies to employ average quality labour to produce a "good enough" result. Ferrari employ top engineers to produce an extremely high quality result.
Ford scales, Ferrari doesn't.