It isn't even just that ARM can't handle the load, the software is designed to run on two different architectures for computer chips. Windows 8 runs on x86 (Intel/AMD) and Windows RT runs on ARM. So there is no update for Windows RT. It is it's own OS (split from traditional Windows) designed to run on specific hardware.
To put it another way outside of computers, it is like diesel vs. gasoline (petrol for our non-US members
). Both are vehicle fuels but you have to use the right fuel in the right vehicle.
JP