I think it’s a multifaceted problem. Police data is being scrutinized more heavily, arrests and forced confessions are being broadcasted widely, more and more people have wised up to the golden rule of don’t talk to cops, and police are tasked with so many stupid/useless things like the drug war, stopping abortions, arresting kids in schools whose parent just died of cancer, that they don’t devote enough resources to actual crimes like murders. Plus, you know, people are lazy in general and cop unions are the best at protecting their members.