Historically, the US Government. NIST mandated that only those operating systems should be used where existing applications can easily be ported to and fro, and

Historically, the US Government. NIST mandated that only those operating systems should be used where existing applications can easily be ported to and fro, and POSIX compliant OS-s tick all the boxes. Windows NT got its POSIX subsystem for this reason: https://en.wikipedia.org/wiki/Microsoft_POSIX_subsystem https://en.wikipedia.org/wiki/Microsoft_POSIX_subsystem