Because people already had a server to run Exchange, which is actually pretty good, and if you’re already paying a fortune for Windows, why not use it?
Linux is definitely not free, you need to hire staff who know how it works and you probably also need to pay a support contract for someone even more qualified where necessary (e.g. Red Hat, who can patch the kernel if that’s what it takes to fix your problem).
Since you’re already paying for both of those with your Exchange server, it was cheaper to use IIS as well. These days Linux is a lot lower maintenance and support contracts are cheaper, so it’s less of a concern.
I wonder if they still even teach windows server in school these days. Back in my days 10ish years ago we had separate courses for windows server and Linux. But when I got a job all the windows server was doing was AD and now even that is either gone or on it’s way out.
Can confirm that Windows Server is taught in school IT programs, and can confirm that Windows Server is still being used for both Active Directory and on-premises virtualization (Hyper-V). I interned at a large international organization with networks on 6 continents and it was moving its server infrastructure back to its own datacenters because of rising costs of cloud hosting. It used Hyper-V on Windows Server to host every thing.
