In some cases the limits on the rudder deflection may also be to address unacceptable handling issues, in which case a load-based devive would be useless. For aircraft where there are configuration dependencies you still need a configuration input.
Load cells are also not that reliable, and especially for hingemoments. One concern I might have is that the hingemoment on a rudder is a powerful function of sideslip as well as rudder angle. It might be very difficult to device a limiting scheme based on load which provide appropriate protection (which can be against sideslip loads on other components, so a low rudder load doesn't mean everything is ok) while also ensuring adequate rudder was available for the various OEI and controllability cases.
Limiting based on airspeed is simple and easily understood, and reversionary modes fairly straightforward.