Because unlike something like the theft act which has not changed since its introduction in 1968, traffic law changes daily almost which is why a certain amount of leeway is allowed.
It doesn't help these days that your local traffic cop now does a 1 week traffic law course, a 3 week driving course and that is it. They struggle to understand the law because they are not professional traffic cops anymore, not their fault, its the fault of the forces who do not consider traffic policing to be important enough or serious front line policing despite the fact that they see more dead bodies and make more crime arrests in a year than the average superstar detective sees in his whole career.
Back in the day every trafficcop had to do a 12 week traffic law course at Maidstone or Chelmsford (I was Essex) do 12 weeks of driver training (Standard, Intermediate and Advanced), Motorway course and be trained and qualified, Speed enforcement, (Truvello, Muniquip, VASCAR and, and, and) along with a whole host of other stuff just to become an effective member of the team and we all had specialist subjects (mine was motorcycles and construction and use regs)
It was reckoned it took about 5 years to become fully qualified. Most traffic cops last about 2 - 3 years tops these days.
It is very different now as I found out recently when I was pulled for undertaking on the M4 and the copper was really upset when I said I would elect to go to court and I would claim and win costs against Police. I would have hated to be the unfortunate first poor sod that got pulled by him after he sped off from me.
Very different now, and a big gap in the knowledge base.
Sorry for rabbiting on....