Many cybercrimes in the US are actually committed by small-time crooks already known to the cops. Yet because of limited skills and resources, no supervisor will let a cop investigate a crime he knows will not be prosecuted. It is not even considered "real" police work.
IMAGINE calling your local police after you have been mugged and being told: "Sorry, muggings are complicated. We don't handle those. Try the FBI."
That is exactly what happens every day when Americans call their local police to report online bank fraud, tax-return fraud...