When the sidearm competition came up, it was the Air Force demanding the change, and as the proprietary agent in charge of acquistion, they got to set the ground rules. It is telling that the Army didn't make an effort in the hallways of the Pentagon to be the choice.
The entrants to that competition did not include any polymer pistols. This was over 25 years ago. Beretta won the contract in as much as the Italian government reminded the US they needed to do that to keep our refueling stations and parachute unit there. The contract requirements were flexible enough in that light.
Nonetheless, the move to double stack double action pistols had been coming since their introduction in the 1930's, same as the move to a small caliber automatic weapon. Plenty had been around since the 1930's too, the battlefield statistics examined, etc. Once the real results of how casualties were created, the result was carrying a weapon that could create more than the enemy's weapon.
Ironically, the Air Force was in charge of that program, too. Why? Because the Army command structure is heavily laden with extremely conservative thinkers who are prone to stick to what they know works, and to stress training as the way to make up the difference.
In reality, the Army has got caught with their pants down in the beginning stages of most wars, and had to fight to a position of superiority. We do that - the politicians fritter it away.
With the mindset like that - don't forget, the chief armorer in Lincoln's day refused to even watch a firepower demonstration of the new fangled lever action that could "shoot all week," it becomes expedient to work around Army Command to get things done. No animosity, just the situation. They don't like experimenting, they do enjoy the benefits when directed.
Don't be misled about how much polymers are making inroads in the design of mass production weapons. It's precisely the same thinking that led to the design elements of the M16 - a drop forged receiver that needs little machining to finish, plus composite parts that provide higher strength to weight ratios than wood with less maintenance. There's a reason the M16 was said to need no cleaning - compared to a M14, it needs a lot less of it, and is much easier to do. It breaks down in seconds with no special tools, and it usually only needs a wipedown to do it. The M1 was notorious for locking up the gas piston if neglected, and it was an armorer task to get it apart when it happened. The M16, not so. User maintenance for the most part.
Looking at the current crop of 1911's, things aren't that good, most of the overly tight civilian models are also handicapped by having to operate with whatever ammo the shooter likes. That creates another difficulty in not having a consistent load to help design the works to operate with. The military issue weapons, not so much. You get one or two loads, it's based in interoperability, the recipe set in stone in comparison. You don't need to experiment with what load might work best in the military, they are tuned to work together and the wrong ammo simply isn't issued.
What is issued is large quantities of it, the methodology of warfare right now is to shoot more ammo, and the other side will have more casualties resulting in the victor having more shooters on their feet to dominate the battlefield. It's very indicative that some small elite units might still issue a few 1911 style weapons - they are likely to need them more because their taskings require it more often. That doesn't make it the primary weapon of choice until the specific situation arises, tho, and they are trained to forecast that, and transition when necessary. It still takes some pretty restrictive circumstances to deliberately choose to use a pistol with half the ammunition capacity, tho. How it operates is really very little to do with it, and the caliber even less so at the ranges it will be used. The 9mm has come a long way in ammo development in 25 years, in either case, tho, military is largely limited to jacketed ball - for penetration purposes. It's the best combat compromise, opposing forces with ammo carriers behind barriers aren't thin skinned targets.
So, for all the conservative dislike of polymer guns in the armies, they are there, they are becoming predominant simply due to economics, and they ARE proven, as the use in LEO and other communities has shown. Why we don't see them in the US is as much a political and Command decision as not dumping the 1911 in the 1940's - complacency and tradition, rather than an open mind to make effective changes.