When I was on active duty in the early to mid 1980s, the 1911A1 was the only game in town. Before my final stint on the Division staff I served as a platoon leader and for a short while as the XO of the Division Pistol Team (Composite Bullseye). I never found our supply of 1911's to be wildly loose or inaccurate and I fired a fair representation of brother officer's assigned pistols in my Brigade when I was at the unit level. On the team we got match grade pistols (including S&W M41's) and ammo as well as Olympic grade coaches. That was a pretty fun time. I never found the 45 to be an exceptionally hard kicker and actually the recoil impulse is more of a push. But it can be pretty heavy and that tends to task the arms and wrists of some female soldiers or folks whose background in life doesn't include a familiarity with firearms.
The big push to change the status quo was blamed on several things:
1. 1911A1's were running on the ragged edge of the life span.
2. There was the issue of training the females and others of limited shooting backgrounds to handle the pistol.
3. But the overriding excuse was logistics to bring our military into line with our NATO allies, the 9mm being the sole caliber of choice for Europe. Of course at that time we were all training to be in combat with the Russians, so that made sense.
However as in everything that ultimately makes a significant change in the TO&E, it came down to the greed factor. Just as when the M16 was pushed over the M14 and before that the Garand and so on, the ever present military industrial complex got geared up to offer up a "needed replacement." It is interesting to note the factors in common with the progression of our military battle rifles: Decreasing caliber size and increasing numbers in terms of capacity. Why wouldn't the same hold true for a sidearm?
The Beretta and the Sig Sauer P226 were the leading contenders. I personally own both (plus 3 1911 styles) and I prefer the Sig. But in terms of the competition the Beretta was cheaper in the bidding process. I don't have the stats from those tests on the numbers of malfunctions for any of the pistols tested, but the "cut-out slide" on the Beretta has to be a factor in reducing the number of stove-pipe style jams resulting from perhaps underpowered ammo or springs too strong on the pistol.
The money for private business is often made in the accessories arena: Sights, ammo, magazines, grips, small parts, etc. Plus I'm sure the manufacturers involved were amping up the lobbyists to push their product lines to the procurement divisions.
There are extremely good arguments to be made in favor of accuracy over volume of fire. Stopping power is quite a nebulous term. Which shot offers more power: A 45acp hit in the palm of the hand or a 9mm on the upper lip at the base of the nose, traveling straight line at over 1,000fps? Accuracy = finality. Some folks might argue that given similar ball ammo, just based on the cross sectional density of the 45 being greater than the 9mm most all hits on the center of mass the 45 will be the better stopper. Well, maybe or maybe not.
A very close friend of mine, just retired as the senior officer in charge of all army special forces education and he swears BY (not "at") his M9. He says the ODA's are equally happy. The USMC seems hell bent on replacing the M9 with the Kimber 45. But how much of that might be a tried and true sense of traditionalism? The marines are very big on their traditions. The idea that Chesty Puller carried a 45 probably carries a lot more weight than some of the other factors. That's just my impression though. I might be totally off base, there.
This is one thing that has me scratching my head, why the military doesn't do more "continuing" contracts, where pistols which are getting worn out get sold on the "used" market, and new ones are bought, on a yearly basis.
As far as the 9mm/.45 debate, really, how often do military people (outside of certain special ops units who do their own procurement) actually use their handguns to fire at an enemy?