Why don't transponders have active and stand-by value like radios? This answer explains how pilots when switching code on transponder should either carefully avoid temporarily squawking one of the emergency codes, or should temporarily put the transponder in stand-by.

Radios (both comm and nav) usually have an active frequency and a stand-by frequency and the pilot selects the stand-by frequency and then activates it with a button, which avoids temporarily selecting different frequency and limits the time when the radio is not receiving.

Why don't transponders have similar mechanism? It seems it would be much more useful for them than it is for the radio.
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.