Police, and the rest of the enforcement & judicial system like any laws that can be enforced that don't require a lot of effort or risk, and can be prosecuted easily. Laws that provide good "return" also are favored. Law enforcement and the judicial system favors laws that have the potiental to make money through fines, property seizure, "favored" lawyers, etc.
This is one of the reason why they gravitate towards traffic laws so much. You rarely ever hear about cops pulling over people red-handed for road-rage or reckless driving. You DO however hear constantly about cops writing citations for speeding, parking violations, etc. Around here, the red-light cameras are a hot topic, and they have been proven not to really increase safety, only to shift the problem to rear-end collisions. They DO make money though! Of course the cops don't want to get involved with a potientally violent person behind the wheel that's in a fit of road-rage. Same goes for the cannabis laws. Police like these becasue typically cannabis users are non-violent, and many of the facts that Brettsomers stated are particularly true as well.
One of my biggest gripes with the sex-offender "witch hunt" is the way it victimizes people. We don't have very clear explanations on this site of what these people actually did, but I imagine, many of them could be "wrong place at wrong times" type crimes. For instance, a parent could be charged with a sex offense these days by taking pictures of their naked children in the bathtub. There was also a case a few years ago about a family that was skinnydipping off the back of their boat. The Coast guard stopped to see if everything was OK (They got a call about a stranded boat) The parents ended up getting charged with sex offenses, and loosing custody of their children!
I work with the Boy Scouts, and it we cannot even as so much as touch one of the boys in ANY way! This can be something as simple as grabbing a a scout's hand to help pull them up a steep hill during a hike, or helping him properly wear safety equipment, like positioning the head strap on goggles. In fact, the local scout camp had to spend a fortune re-engineering their showers becasue the leaders couldn't shower in the same facilities with the kids.
While I am all for protecting kids from these predators, the way these wich hunts for sexual predators are going on, everyone in this entire nation is going to end up on this list for some reason or another!