It was a requirement on government contract tenders. If you didn't support POSIX, you didn't get the contract and the government would have continued using SunOS or AIX or something instead of Windows NT. Remember that POSIX was designed to make these OSes somewhat interchangeable - they wouldn't order something they knew was incompatible with everything they already had.

Exactly, thus UNIX/POSIX being everywhere that mattered, alongside the language created to make UNIX portable, C.

For those that don't read standards, POSIX certification requires the existence of a C compiler, the more recent one being C17 compliant.

https://pubs.opengroup.org/onlinepubs/9799919799/utilities/c...