I read once (actually, in many books and articles) that the US Army's cartridge and weapon choices this century has always been heavily influenced, perhaps too much so, by long-range competetive marksmen rather than by actual grunts. Considering the .30-06/.308 dynasty, this seems plausible enough.
I've always agreed with you that we should have commenced development of an intermediate-sized cartridge and rifle following WWII (perhaps even preceeding it). I recall reading that an Army colonel, Renee R. Studler (what a name!) was the man who stood directly in the path of such efforts, to the point of forcing NATO to accept the .308 cartridge AND the Army to reneg on its promise to adopt the FN-FAL rifle. You probably know more than I about that story.
From what little I can glean at this time, it seems that the 6.8mm was designed by the grunts who'll actually USE it, and not in some lab or by some advanced hobbyist on a controlled, clean, and safe range. Better late, then, than never.
Wasn't it Churchill who once said words to the effect that, "Sooner or later, the Americans do the right thing.".
OH! yes...He was also one of those who tried his hardest to kill Armalite/Stoner's AR-15, as well. See Blake Stevens and Ed Ezell's The Black Rifle: M16 Retrospective for more on Studler and the M16 fight.