It's because at the bottom line, people don't respect what police do, and that's a lack of respect for the law. If you don't respect the law, you aren't going to respect its enforcers.
For better or for worse, people simply don't feel that most traffic infractions should be against the law. Because the vast majority of most folks' encounters with police are the result of traffic violations, they aren't going to respect the police. Likewise, I think that people really don't respect the idea behind the War on Drugs. I think that most people would agree in theory with the concept that drugs should be illegal, but they don't respect the means by which the drug war is fought.
Bottom line: people don't like speeding tickets, so they don't like cops.
Most police departments have turned into Revenuers, that is, their goal is the same as most guvmint entities, to protect its turf, get newer and better "stuff", and grow its budget. As a result, you have the growth of photo radar, even shortening the yellow light duration in some jurisdictions to give more tickets. Rear-end collisions have gone up where they install these things. And the companies that market the systems to the police departments market them on the basis of how much money they can make off them. Meanwhile the police spinners lie to the public about how they are concerned about safety.
Meanwhile, in most jurisdictions, most police departments release illegal immigrants, even when they already have them in custody over some minor traffic violation.
The public is pretty saavy. The public does not want to allow reckless drivers and supports taking them off the streets, but when the traffic in most areas flows over the speed limit all the time, and the radar revenuers come along and issue you an expensive ticket for keeping up with the traffic, slightly over the posted limit, yes, most rightly resent it, and know that its "just about the money". Particularly when they happen to net serious criminals in my example, and they have a catch and release policy. Our Constitution was written by folks who had suffered a tyrannical guvernment and were trying to protect against one again. We may not yet have a tyrannical gubmint, but it is a slippery slope and a process of increments, as is Liberalism.