Good luck certifying an automotive or an industrial system that can lead to loss of life or severe disruption if it malfunctions. As far as I am aware, QNX, VxWorks, and CMSIS have all been used in systems that passed certifications. As IoT &co. get applied more broadly in our society, they are put in more places where they are expected to function flawlessly. I would rather want my power grid or water utility operator to use CMSIS, for example, than rt-linux.

The only serious user of rt-linux which uses it to build a hard-RT system that I know is SpaceX. Look up the Reddit AMA post from them on how rt-linux is used. They are staying on a specific kernel version, had to do kernel patches (other than applying rt-linux), they use a very specific model of an old Xeon for low latency interrupts, incorrect programming of one process can disrupt the RT behaviour of another process, and most importantly, they have a secondary board, a truly real time one (PowerPC based, IIRC) to communicate with the real hardware.