Because this is generally not secure. The driver OS will have access to hardware that can bypass the memory restrictions set upon it by the microkernel.
There is sometimes special hardware to address this but they are too complex to manage from the kernel.
Most OS's aren't designed to be that secure, though. That's why I wonder why it hasn't been tried more for usability. A security-focused setup certainly has more to be concerned about. Like I advocated with Xen, a good start would be making the host OpenBSD. They should be able to get hardware that would be compatible.
Because this is generally not secure. The driver OS will have access to hardware that can bypass the memory restrictions set upon it by the microkernel.
There is sometimes special hardware to address this but they are too complex to manage from the kernel.