Because the military is slow to change, and like I said earlier US forces generally share log-trains and outpost locations with friendly forces, so it's a lot easier on everyone if there's caliber commonality between everyone. Honestly the only two pistols I could see making headway in the trials, that are an actual improvement over the M9, would be the FNX9 or CZ 75B or BD although the military would probably opt for the 75BD because of the decocker. But because of US Mil brass seeing CZ as "well they used to be commies, they couldn't possibly be good guys now" I doubt the CZ would even make it into the trial rotation.