The US Army is about the least innovative organization around. It is incredibly conservative towards equipment. It likes what it has, it wants something better but only a little different, and totally rejects revolutionary change. Sticking something like a pistol grip on a stock was too much for the Army to stomach in the 50’s.
Another issue is that the Army likes dealing with the same big established companies and in time the hiearchy represents the financial interests of their contractors. The Army does not like changing vendors and product changes require changing vendors or dealing with new one. Retired Generals are guaranteed a Corporate paycheck as long as their loyal Deputies keep funneling money to the company that hired their Ex-Boss. General/President Eisenhower called it the Military Industrial Complex for good reason. Once the Army is locked into perpetual sole source you find it is not in the interest of the Army or the Corporation to introduce change.
Large corporations, as it has been noticed, are not good at innovation. They are good at stealing good ideas, but not so good at internal innovation:
http://blogs.hbr.org/cs/2012/09/why_big_companies_cant_innovate.html
As you all have noticed, the good ideas in firearms all come from individuals and small businesses.
As you have noticed, in your lifetime, the software and electronic industries were built up by entrepreneurs, like Bill Gates and Steve Jobs, guys who started these mega industries from their garages. In time the entrepreneurs will leave, the “suits” take over, and the big corporations stagnate, but as long as they are propped up by taxpayer subsidies, they never go away.