Earlier quoted context omitted.
I bet their billing system is custom and was developed in times when Windows was a better choice. And nobody was able (or even tried) to justify a switch since then.
At what point in history was that? Unix(tm) and mainframes predate windows server by a far margin and was never seen as inferior to windows by the wider sysadmin community, there might have been a time when windows server had merit as the discount solutions for people who could not afford highly available Unix boxen, but it was never seen as superior or particularly well suited for critical infrastructure. I think th…
*nix OSes may have been as far superior to Windows as you can imagine, but the development ecosystem certainly wasn't. There were Delphi, VB and other RAD tools for Windows, with tons of GUI and general purpose libraries and toolkits. Bun on *nix you basically had just GCC and vim/emacs. There were no GitHub, no npm, no AWS, nothing. If you'd compare TCO of an enterprise app plus necessary infrastructure, the Windows-flavored one would come a clear winner. Even mixed environments were too much of a hassle, due to poor interoperability.