Why limit rudder deflection based on airspeed?
It's because, everything else being equal, dynamic pressure is proportional to TAS squared. Consequently, the large rudder deflection needed to bring Vmca and Vmcg down to usable values might cause structural failure or control problems when applied at Vmo.
Not every transport category aeroplane needs speed referenced rudder travel limiters. ATR 42-300 is a bit short on power so it has no limiter at all and DHC-8 400 uses mechanical limiter connected to flap handle. However, I don't think that such a workaround would be practical on jets.
I'm not sure how rudder works on other FBW types, but, apart from sophisticated yaw damper and travel limiter, there's nothing FBW about A320's rudder. It's controlled via cables and hydraulically operated.