It wasn't really an option when they started. Nginx couldn't and still can't run ASP.NET application. You could maybe put the application in a container, or run it as directly with the new versions of .Net, but that requires reworking the entire application. There's not really a lot to gain, not if you already know IIS.
I wonder to what degree they have automated the configuration of IIS…? From my POV it seems really tricky to automate. But I am by no means an IIS expert, I just have some clients that use it.
You can automate IIS configuration with Ansible, that's what we do.
Mostly the issue is documentation, you sort of have to know what things are called, but you can get around that by using win_iis_webapppool and get the configuration from a running IIS server and then work out which properties you can set.
There might be things you can not automate, we don't use that many advanced features.
Edit: I wasn't being fair. Ansible, is actually pretty good at managing Windows servers. It's just different from managing Linux servers.
