ST in STMcube prefers FreeRTOS, Keil his RTX. Unfortunately, for example STM32F1 not exist drivers compatible with CMSIS 2.02 and so there is the question of whether to use RTX or FreeRTOS? It is one of them significantly better?
Proper reason?
"Great interface, portability, MPU support, quality code." is way too short to actually be a proper reason. That sentence doesn't say anything about how great the interface is for the other OS, or what difference there is in portability or MPU support or if there is any difference in code quality. And it doesn't say what actual needs that resulted in that recommendation.
But back to the beginning - you haven't written any information giving the indication that you actually need, or would get any advantages from, using an OS. With zero needs documented by you, people can't give suggestions of what would be a better or worse OS or if you would be just as well off without one. So why complain about lack of reasons from someone else when you didn't start yourself?