One of the main reasons is that the number of offences that can be committed by a motorist is immense. Compare this to (say) a mugger or a paedophile. Those miscreants really only have one activity that makes them fall foul of the law and it is much easier for them to remain on the right side.
Of course, if you are talking about persecution, there are far more motorists than muggers and (here�s the rub) it is much easier for police to detect and prosecute the numerous offences which millions of people are likely to commit than to detect and prosecute one offence which far fewer people are prone to. The evidence to support most motoring prosecutions is straightforward and readily available. Furthermore motoring offenders (or at least their vehicles) are easily identifiable, having by law to carry a registration plate. Detecting and prosecuting a mugger, wearing a hoodie and who escapes by running up a back alley is not so easy.
So the next question is, where should the police be concentrating their efforts?