There is actually a really strong use case for 6mm ARC (or something similar) in Recce type rifles. There is an awareness that the standard issue rifles really underperform starting at 90 - 110 yards, because the ammo no longer consistently expands, a byproduct of velocity loss due to short barrels. So having a "transitional" set up that is still light weight but delivering ~80 grain expanding rounds at 2200+ FPS has a lot of value.
Which is not the problem you are generally trying to solve with U.S. LE sniper rifles. There you need accuracy, dependability, and bullet construction that lends itself to ignoring glass and other light barriers. 6.5 CM doesn't move the needle on any of the things you need (less recoil maybe?), until you start discussing engagement distances of 300 Y +. And that just isn't a thing, mostly because you can't positively ID at those distances (usually), you're in urban settings (usually), the subject is in/near a residence (usually).
But the problem is that everyone is doing long range cross training, or trying to "stretch their legs" at distance, and realizing that in terms of "hits on steel", 6.5 CM is a better option. But that is also how the German Artillery in WWII ended up carrying long barreled 9mm Lugers instread of rifles - they were getting similar hit percentages at distance. No one stopped to check if the bullet was performing as demanded. .308 has a long and documented performance in this role. It may still be that 6.5 CM is an adequate choice, but you can also argue that .300 WSM, or 7mm-08, or a million other cartridges need to be looked at.
