Their reason? The Army worries that the 5.56mm round fired by the M27 -- and by the M4 and the M249 -- will not penetrate modern body armor used by potential adversaries:
Last May, Gen. Mark Milley testified to the Senate Armed Services Committee that the service's current M855A1 Enhanced Performance Round will not defeat enemy body armor plates similar to the U.S. military-issue rifle plates such as the Enhanced Small Arms Protective Insert, or ESAPI.The cleverly named ESAPI is known to the troops as the "chicken plate," a ceramic plate inserted in the front and back of standard body armor to provide added protection because our adversaries use weapons that fire 7.62mm rounds, larger and more powerful than the 5.56mm round fired by the M4 and M249. Naturally, then the Army wants to develop a rifle that fires a 7.62mm round. Oddly enough, before the Vietnam conflict, all U.S. infantry small arms used a 7.62mm round. We switched to conform with NATO because the European nations didn't like carry that kind of weight around. But I digress.
Because of the extra weight involved, many U.S. troops, who already carry close to 100 pounds of gear most of the time they leave base, decline to insert the chicken plate. In our current conflicts, you are far more likely to be it by an improvised explosive device than by a random shot from an untrained jihadi who happens to have a higher-caliber weapon than you do. Who made those weapons (and the body armor the Army is worried about, which the jihadis don't wear)? The Chinese and the Russians, of course.
I don't have any problem with the Army wanting to return to the 7.62mm round. I just suspect that it is because Marines can hit their enemies where the chicken plate isn't, and the Army can't. Just sayin'.