Or because they had equipment contracts or supply deals with IBM who were keen to use an inhouse product wherever possible. The mainframe computers and larger disk and network controller devices that populated the datacenter generally came with a configuration console running OS/2.
I used it in the 90s for a quite complicated develpment (multi channel audio recorder for call centers).
We didn't choose it because of any contract with IBM or anything like that, but because the alternative was Windows.
I didn't quite like it (at some times I hated it actually) because of so many absolutely insane design decisions by IBM. It even had stupid bugs due to the coexistance of 16 and 32 bit code. I remember one particularly well, passing a buffer that crossed a 16 bit memory boundary to the send() or recv() functions in the socket library
caused memory corruption even in 32 bit programs.
Memory management was pure crap, there was absolutely no serious performance monitoring software, the design had such horrible flaws that removing the diskette controller and driver made shell pipes (using "|" in the shell) stopped working... I could write pages and pages.
But at the time (and even adding a small hardware supervisor that hit the reset line in case OS/2 hung because of some UI event queue idiocy, which managed to block even some background operations) we achieved uptimes in excess of a year. We usually only stopped
the recorders when the DDS tape drives needed a replacement. Also I must admit that the API wasn't bad for concurrent programming. We ran hundreds of threads and the toolset was fine.
You can like or dislike IBM (actually I have banned IBM hardware at work recently thanks to their great support with a defective SAS controller) but it was really shocking that people who are supposed to know how to design operating systems produced that piece of crap.
But again, Windows was remarkably worse. If I remember well, the Windows versions from the 90's even had some serious uptime limitations because of some counter overflowing. And of course at the time doing silly stuff like changing an IP address required a Windows reboot, even on NT.