Why would true crime be limited to that demographic? Lots of different people have an interest in certain crimes, particularly unsolved or controversial ones.
No one is saying that it's limited, just that it's the primary demographic that comes to mind when true crime is the subject.
What I learned is that true crime as a genre is different than the True Crime Community(TCC) which is a different demo and is associated with glorification of violence and one of the sources of enacted violence.
