Tradition is a large part of it. It is like the story of the Royal Army going over their crew drills in the 1920s. They had to call in some really old decrepit veterans to tell them what some parts of the drill meant. In one rather telling case, during an artillery drill, two soldiers were posted on either side of the field piece during the drill, and just stood there the whole time, doing nothing. When one of the old vets was queried, he told the assembled staff officers that the two soldiers were there to "hold the horses." Large organizations often have wierd rituals that no-one can effectively trace or explain. Anthropologists could have a lot of fun in a medium to large-sized police agency, figuring some things out. The halmark of a good organization is the ability to analyze this kind of stuff and easily discard it.
Our agency, at least, has gotten rid of many 10-codes and uses more and more plain text. Some codes are still used, however, for brevity and clarity, and we also have several non-standard codes, which change periodically, for sensitive information and officer safety issues. Bad guys know the "standard" 10-codes, too, and I have seen a few situations go south when the radio suddenly blares "your subject is 10-99 (wanted)," and the bad guy knows exactly what that means.