It's pointless to introduce new calibers or systems unless it is enough better to justify the expense of conversion.
A civilian here and there will buy anything any one else will sell, even if just for the novelty of it.
But services and agencies are considering their existing infrastructure, part's, mag's, armorer training, etc.
Look at the Johnson rifle.
Except for the bayonet issue it was just as good as the M-1.
But the Gov already had stock piles of Garand parts, training manuals, and clipped ammo, none of which was any use to the Johnson which would have required it's own versions of these things.
Semi auto pistols are another example, until they started offering significantly more rounds, not just 1 or 2 PD's found it more practical to just stick with revolvers.