One of the problems is that the Linux RT kernel doesn’t stop you from using/writing devices/drivers that break those guarantees systemwide, as I understand it.
A kernel alone on a random hardware platform can't, but a kernel and a hardware core that watchdogs and flips the bird at a rt-violating driver+device, sure (I've toyed with VxWorks on devices that work this way).
Usually when you're entering that realm of such requirements, you're rarely talking about off-the-shelf devices.
One that has no such drivers? Which would then imply limited hardware support as the price for the guarantee, which is a trade-off a general purpose kernel like Linux isn't going to make.
