An explanation in a non-legal context might go as follows:
If a magazine capacity limitation effectively limits the capacity of criminals to commit violent crime with a gun then that same limitation on the law abiding MUST neccesarily limit their ability to defend themselves against a violent crime. Both offense and defense with a gun are governed by the same rules. If a law handicaps one, it must handicap the other. Now, we all know that criminals won't bother with the law and most law abiding will, eventually. But, for the thought experiment it should be considered plausible.
Also, why do the police get normal capacity magazines and assault weapons if these weapons serve no legitimate purpose for defense? Do they face some special type of criminal that we, the citizen do not encounter? Are there lives more important than ours?
Do normal capacity magazines present such a danger that it's crucial to the interests of the state that they be banned? Are "assault weapons" so dangerous that they must be banned but functionally identical weapons without the scary COSMETIC features OK for us to own? A law, especially one that is infringing on a constitutional right, may not be arbitrary. It must be absolutely crucial to the interests of the state and it must be as narrowly tailored as possible. A hard boat to row for the state in all of this banning nonsense. Now, let's hope there are five justices that are honest.....