As the sibling said, I doubt IIS is only used for legacy systems. My client has a brand spanking new system that only runs on Windows, and the "web" parts of it

As the sibling said, I doubt IIS is only used for legacy systems. My client has a brand spanking new system that only runs on Windows, and the "web" parts of it (HTTP APIs, really) are running on top of IIS. However, I'm unable to answer the "why" part of your question, though I'm very curious about it. In their case, nothing they do couldn't be done better on Linux. Windows has actually been a complete PITA for deployment. But apparently, the provider, for some reason, has "always done it this way".