Just a WAG: Conservatives.
This affects nearly all professions, not just policemen, but the example I am most familiar with is with teachers.
In the early 30’s, Pavlov and Skinner revolutionized psychology with Behaviorism, and there was a move to push Behaviorism into the classrooms. Teacher training programs were changed, teachers had to learn the new systems, etc. By the 60’s, Behaviorism became the dominant teaching style in the US.
However, as early as the 60’s, it became clear that Behaviorism had severe flaws, namely that brains don’t work like that in terms of learning. Speech, for example, uses different brain processes than simple reward/punishment, and learning disabilities (also immune to Behaviorism) were being identified.
So, from the 70’s until now, new teachers have been questioning why they have to teach a certain way, when it is clearly wrong. However, the older teachers do not want to learn new techniques or take new certification tests. The problem is that older teachers have the power, and are usually the senior teachers, administrators, and union reps.
This creates a cycle that by the time the new teachers gain the power, they’ve become old teachers, and the cycle starts anew.
I can imagine the same thing happened with police. When they were young, they questioned the use of outdated technology. However, by the time they got the power to do something about it, their mindset became, “If it was good enough for me, it’s good enough for you, rookie.”