All militaries are reluctant to change, and they are more reluctant to change when it comes to equipment and technology. Plus, let's not forgot politics. A defense contractor will do his darnest to make sure that his weapon will work and kill the enemy, but if his company happens to be in a district that has poor representation or his competitor's retired military advisors have more pull in the national legislature, then he is going to have a tough time getting his weapon sold. Granted, some of the best weapons in the world have finally made it to the field, but some--the F-111, the F-14, F-117, the Osprey--had serious delays because of politics. Some like the A-12 and the Sgt York were thankfully killed before they got people from the home team killed. Remember, the stink Admiral Rickover had trying to convince the Navy to go nuclear? Still some haven't made it to the field such as caseless rifles have never seen the light of day.