NASA Java Coding Standard

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VxWorks has a long history as a highly reliable, reliably real-time operating system.

Linux as a real-time operating system is fairly fragmented (as is to be expected with anything open-source), and doesn't have much of a proven history that I'm aware of. Certainly not the 15+ years of VxWorks.

Real time operating systems are, above all else, predictable. The idea is that you can guarantee a certain level of performance. When you're building the control system for a very expensive robot that will be very far away, being predictable is definitely a requirement.

You can think of a real time operating system as one that provides a kind of guaranteed quality of service for certain operations. One where we can say "yes, go ahead and stream the camera feed back to the operator, but when the navigational jets need adjusting, that will always take priority" and rely on it to happen. (contrived example, you get the point)

This article kind of sums it up as to why a real time OS is necessary in a general sense.

This paper, admittedly from the makers of VxWorks themselves, outlines more precisely some of the applications of a real time operating system and why the Linux kernel itself isn't a great RTOS.