NHTSA provides substantial grant funding for traffic-safety enforcement, conditional on the department's enforcement statistics for a particular set of safety-oriented violations, especially seatbelts. The brass encouraged us to take a minimum number of enforcement actions to maximize the chances of having the grant expanded. (It paid a for substantial chunk of our salaries and offered regular 1 1/2 overtime.)
A more likely explanation is the severe underfunding and understaffing of prosecutor's offices and state court systems. Most prosecuting attorneys are responsible for carrying hundreds of cases at a time-- they have no choice but to prioritize, aggressively.
(I don't think it alters your point at all because you're still talking about constrained resources and a need to prioritize, but in traffic court in my village the PD is the prosecutor; my negotiation was with a police officer and he in turn represented the results of that negotiation to the judge. Same with everyone else's tickets. Only more serious stuff involved the town prosecutor - he was handling code violations, underage drinking, and a DUI.)
Pretty much every traffic officer I know could find at least 5 reasons to pull over any car, no matter how scrupulous the driver.
