Take it apart and think.
1) it's all steel - a gun that is going to get literally dragged across the beach, endure mud, and then be called on for use after the primary weapon was down needs to be extremely durable and reliable. Like it or not, polymer pistols do that, all steel won't. Nothing on a polymer frame can oxidize or rust, so reduces the maintenance load on the carrier.
2) It's a .45 - no longer a compatible NATO caliber. Adding a different caliber to the wartime logistics chain is another incremental load on the Army that does it. When you and your Allies all share the same calibers, there is a higher incidence of cross leveling ammo and getting enough to go around.
3) It's expensive to make - all steel/alloy guns require 100% machining, other than what small parts you can make casting or in MIM. Molding the polymer frame reduces the cost 25% or more - which we see when making a choice for a firearm retail. If the government needs to buy a firearm, why limit the contract when you can get 25% more guns for the same price? Why punish the taxpayer with a high cost firearm when a polymer framed one does the same job?
4) The 1911 is a .45 - which becomes a difficult proposition creating a double stack grip that can be handled by smaller soldiers. We may provoke catcalls about somebody serving needing to be in shape and capable of doing the job, the reality is that in modern times, the average woman is the same size as the pastime soldiers of the 1940s. If a modern female Marine would be better served by a smaller firearm in 9mm, her male counterpart of 75 years ago would too. (Which is entirely the point of the M16.)
5) Since the auto pistol as an issue gun is actually a badge of authority - then why accept the 1911 .45 as the premier example? Are there not others more deserving to signify that the bearer is one with authority? Why not a gold plated Beretta M9, or better yet, some other more exotic PDW? It's all the sidearm really is, Why not an HK MP7, or SIG MPX, or Magpul FMG-9? If you had a choice, would you want a SA pistol with 7 or 8 shot magazine, or a full auto submachine gun with 32 round mag?
It's not what you think would be cool for someone else to carry, what would YOU want dragging YOUR web gear across the beach?
Nobody chooses less capable tools when their life depends on them.
Now there's something to discuss, take a big deep breath and give it a moment before going into attack mode. And, BTW, I'm selling my Glock to buy a P938, so don't get to your drawers into a twist.