The whole military wont switch cause something like the 6.8 is not a nato round. Its not a cheap worldwide manufactured ammo that our allies can use. Same reason the military switched to the 9mm. The 45 was time tested manstopper(no im not starting a 9mm vs45 arguement) but the 9mm was a nato round.
Who cares what kinda of guns, caliber or ammo the world carries. Cost shouldnt be as big as issue as it is. The military drops million dollar bombs like they got it to burn. Why not invest in a better platform and round. Try the 6.8, try the 6.5 G assualt rifles, hell give em all 308 battle rifles.