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 that's kind of whats in play here, few finance systems were designed as critical as well finance was that department that generated a few reports that nobody really read and rarely interacted with the core productive business to the point where flaws/bugs in finance systems could have an real impact until very recently so what you have is a bunch of aging discount/non-critical system that have been promoted to critical infrastructure nearly by accident.

The same goes for the desktop support infrastructure build around AD and SMB file shares, where systems that used to be auxiliary nice to have for clerical workers ended up as critical for the actual productive divisions without ever receiving any real hardening.

That was until 15-20 years ago.

*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.

Anecdotal, but working in retail in the 90s, the backend system was some *nix server, the wireless Telxon guns ran DOS 3.2, and there was a dedicated OS/2 Warp desktop that would perform reporting and aggregate data.

I am not sure what the registers ran, but they were IBM, all text-based.

Also, I remember my friend was a manager at AMC theatres at the time, and their system was entirely text-based. I remember watching him run end-of-day reports and you could see it zip up the files and hear the modem dial out and connect to corporate.

Dont make the mistake of thinking that the crap that was the pure gnu toolchain was in any way equal to what was available for those who could afford to run commercial unix(tm), it was an completely different world that Linux would not catch up to until the 00's mostly by absorbing frameworks and software from the world of commercial unix(tm) and from more recent developments.

I dont think i can recall any commercial Unix even shipping gcc back in the day.

Thick desktops were ruled by Windows so if you wanted a custom application it was Microsoft Visual Basic or Visual C++, Java as desktop apps didn't seem to catch on for the enterprise. Even if you tried to bring an application to the web back in the 90s you could still end up caught in the Microsoft ecosystem via ActiveX.

And desktops were never critical infrastructure were almost exclusively talking about servers here not disposable client devices as there is no such thing as an highly available desktop/laptop.

I have personally been involved in more java fat client then activeX based fake webapps, but im also not an wintel admin with an background in non-critical mid sized business applications.

Depending on the version of Windows, it may not have even had native TCP/IP support by default.

Trumpet Winsock was a very important piece of software for a bit, from Windows 3 to 98.

Serial ports were much more popular back then.

Microsoft Visual Basic and Visual C++ had quite the market hold when a lot of this was built in the 90s

If you're going to have a billing system that you need to rely on for the functioning of your business. You're going to want to have ongoing support for that system, this is why so many enterprise options are Windows based. Especcially in earlier years, and financial suites, once implemented, are very rarely ever changed. If its still bringing in money, the higher-ups dont care.