In 1999 my team at Williams Electronics chose XINU as the O/S for the Pinball 2000 project:

https://www.youtube.com/watch?v=8rHbsCyde1g

At the time, embedded X86 boards were not really a thing yet so we needed something simple that could run with a COTS PC motherboard.

The major factor was that the licensing for other operating systems was too costly or too restrictive. Anything GPL-based was ruled out by our lawyers. Turning Linux into an RTOS would have forced us to release source code and that wasn't a fight with Legal that anyone had time for.

XINU turned out to be just what we needed. The process limit wasn't a huge problem since we were used to running threads on a single process. Adding networking later turned out to be very easy.