There's a very good treatment of these issues in the late Christopher Stuntz's book, The Collapse of American Criminal Justice. Basically he blames a mixture of excessive criminalization/sentencing (eg drug laws), plea bargaining, and lax policing which prefers arresting people for easy-to-convict crimes - not least because law enforcement officers nowadays rarely live in the same communities where crime occurs, and so lack respect from, sensitivity towards, and intelligence about the communities that they police.