1) Because some units in Spec-Ops may, or may not, use a product is ZERO indication of it ever making it into the hands of the common, hard working soldier, marine, sailor, airman. Special Operations are "Special" for a reason.
2) Picked up a paper or seen a news broadcast lately? The economy is shot to hell and every state, federal and most local governments are over budget and cutting programs.
To think that the government is going to okay billions of dollars in spending to replace one rifle with another that do the same thing, across the board, is pie in the sky dreaming.
6.8spc isn't a common round. No advantage to retooling Salt Lake to load millions of new rounds. It's just to expensive and lacks extended range that the 5.56mm already delivers.
6.5 Grendel? See above, but it does offer better range
There is no immediate advantage to retooling and retraining our entire military mid conflict, with elections looming, there is about a 5% chance, at best, this comes to pass.
Unless you privatize the entire military model. I wonder what Machiavelli would say about that?!