Why don't transponders have active and stand-by value like radios?
For navigation and communication radios the feature of having a second readout that can be pre-set to the next frequency is useful since the next frequency is usually known in advance. Your next transponder code is a somewhat random number and the moment that you will get it is not as predictable as a frequency change. Therefore there is no need for a pre-set / standby squawk code in a transponder.

Also, in a communication radio, when you pre-set the wrong frequency and cannot establish communication after the switch over, a single button press will bring you back to the original frequency. This prevents prolonged communication loss. This is similar in navigation radios. 
For a transponder such a feature has no value, you won't switch back to the previous squawk code. 

The problem of inadvertently selecting a special code can be addressed by other means. For example a digital interface that requires confirmation of the selected code before it is activated.