If you had to design an HA-capable OS from the ground up, would you start with a single executable environment? In this simple, high-performance design, all OS components, device drivers, applications, the works, would all run without memory protection in kernel mode.
On second thought, maybe such an OS wouldn't be suited for HA, simply because if a single software component were to fail, the entire system would crash. And if you wanted to add a software component or otherwise modify the HA system, you'd have to take the system out of service to do so. In other words, the conventional realtime executive architecture wasn't built with HA in mind.
Suppose, then, that you base your HA-enabled OS on a separation of kernel space and user space, so that all applications would run in user mode and enjoy memory protection. You'd even be able to upgrade an application without incurring any downtime.
So far so good, but what would happen if a device driver, filesystem manager, or other essential OS component were to crash? Or what if you needed to add a new driver to a live system? You'd have to rebuild and restart the kernel. Based on such a monolithic kernel architecture, your HA system wouldn't be as available as it should be.