RTOS stands for real-time operating system, versus the general-computing operating system (OS).
The key difference between general-computing operating systems and real-time operating systems is the need for " deterministic " timing behavior in the real-time operating systems. Formally, "deterministic" timing means that operating system services consume only known and expected amounts of time.
Now coming to engineering team decision of selecting an OS depends on many factors i.e. cost, scalability, performance, support, ease of use and most important is the expertise of engineers.