Industry-specific applications often run only on Windows: Small organizations make them, limiting capacity to support more platforms, and their clients are businesses and therefore usually use Windows. For example, I'd expect that most software for running accountants' offices would run only on Windows.
A big exception is vendor-hosted browser-based software, which can run on more platforms. Some features could still be Windows-only. Another exception is phone-based software.
A big exception is vendor-hosted browser-based software, which can run on more platforms. Some features could still be Windows-only. Another exception is phone-based software.