This topic is, not unexpectedly, somewhat able to be attributed to the topic of ROI. And if you know me, I'm all about the ROI.
A few decades ago, there was a performance difference between 9mm and .40 and .45. Now the technology of the round performance has pretty much closed that gap. The overall data from macro market analysis can be found in the FBI data sets; they are the main repository for that kind of info. You can look at individual departments, but most "average" sized departments really don't have many shootings at all. We have had two in the last decade; the suspects died; we use 9mm.
When performance is reasonably assured, you can turn to other topics to be the "tie breaker". Things like round count, officer comfort with the weapon, and COST of ammo (a big concern for any department).
Most everyone here has contributed good points in consideration of this topic. And they all point to this; ROI. The ROI is now about things OTHER than round performance, because that gap is nearly non-existent now.
Sound familiar? The gap between dinos and syns has closed to a point that in "statistically normal" use, you'll never know the difference. So cost and other outside considerations can now be a reason to shift the ROI.
Same goes for ammo today. Statistically you're not going to see any significant performance differnce, so the departments are now looking at other criteria for the decision points.