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Not mutually exclusive: http://toastytech.com/guis/qnxdemo.html

A RTOS can be a general purpose operating system.

Whereas an operating system that isn't an RTOS cannot be general purpose, as it cannot be an RTOS.

Why would you run a desktop OS on an open frame computer?

And before I get downvoted (and lose the precious karma points) into oblivion, I answer the 'question' myself:

- availability of the software

- ease of use

- familiarity

- performance (throughput, not latency)

- popularity (with the advantages that this brings: availability of help, applications, thoroughly tested, etc.)

Many use RPi as a low power, somewhat feeble desktop computer but that doesn't make a desktop OS an ideal or obvious choice for embedded applications where open frame computers are (arguably more sensibly) used.

There is actually a closed source desktop variant called BTRON spec. uITRON clones are just small(and among very few of actually practical) parts of the grandiose TRON Project universe.