Apparently they want to get away from direct impingement and go to a piston but don’t want to upgrade to 7.62 that’s already available.
There are tons of piston AR-15’s out there, many having been produced for some time. And most of them are 5.56 . There is no reason to go to 6.8 except for novelty. As others have stated it’s not significantly better at longer ranges than 5.56- the initial impetus for this program.
CC