When I moved from military to civilian policing in the 70's, a good percentage of officers were Vietnam War vets. There were even some Korean War vets and a few WWII vets. Many of those folks who had encountered people who wanted to kill them believed competence in tactics and weapons was important.
We saw emergence of interest in stopping power, widespread implementation of SWAT teams, officer survival training, high capacity firearms, body armor, and more extensive recruit training. The 1972 amendment of the Civil Rights Act paved the way for women in policing which caused a cultural shift in police recruitment and training. The gold standard police recruit was no longer the big strapping male who could hunt bears with a switch.
We eventually ran out of war veterans and the percentage of recruits with firearms experience dropped off.
Training novice recruits, many with diminutive stature, with firearms is challenging. Training novice recruits with big guns with heavy recoil, report, and muzzle flash can be nearly impossible. Think flinch. .38 special revolvers and 9mm mitigate this dynamic. Magnum revolvers or 10mm or even .40 cal not so much. This, the cost of ammunition, and wear and tear on firearms more than anything else drove the resurgence of the 9mm. Fortunately bullet design has made the 9mm more effective. ( It should also be mentioned that bullet advances in the 9mm have also been applied to other calibers, like the 45acp.)
Basic police academy firearms training is fundamental with basic passing standards. More recruits will do better with 9mm than with heavy calibers.
Follow on training occurs in service within the agency. There are about 17,000 agencies in the U.S. with a full range of emphasis on firearms training. Firearms training is expensive, in terms of staffing, overtime, equipment, and ammunition and is often limited by facility availability. And depends upon emphasis from leadership. But ultimately biudgets are the controlling factor.