It's because the joint failure generally occurs on the side of the tube that is in compression when the pedal is loaded.
Consider the pilot's right foot pushing on the right rudder pedal: That push places the forward side (closest to the firewall) of the pedal in compression. At the same time, it places the *aft* side of the passenger's rudder pedal in compression, because the rudder cable is connected to the bottom of the passenger's right rudder pedal.
The same is true for the left rudder weldment, but in general, given that most of the time only the pilot actuates the pedals, and most of the time the pilot is in the left seat, these failures will show up first on the right pedal weldment.