Well, since I'm one of the guys shopping, let me try and answer.
I'll approach your question in two parts: Why not a 5", and why not a 3".
First, Why not a 5": I have two full size 1911's, and routinely carry one of them. I really like a 1911 as a carry gun because of it's trigger (easy to shoot well under stress) and it's thinness (it really is the most important dimension in concealing). I carry my 1911's strongside IWB with a cant, but I'd like to carry OWB. When sitting or bending over, the 5" gun's muzzle will push out on my pants well below the hem of my t-shirt. The muzzle sits 4" below my belt (measured on my holster with my prefered cant) but if I back the muzzle up 3/4" I get between 3/4 and 7/8" rise towards the beltline (depending on exactly how the holster is cut) And this rise happens to be EXACTLY the dimension that is annoying me. YMMV. In addition, I've had a mid size RIA that pointed, and transitioned between targets better than my full size. In my hands, losing those couple ounces from the muzzle led to a gun that was appreciably faster to draw, aim, shoot, and transition. So easier to conceal, and easier to shoot well. Win win.
Now, Why not smaller: First, while many 3" 1911's are reliable, it's worth noting that a 1911 works on a balance of spring forces and movement speeds. Slide speed and weight, mag spring tension, mainspring tension, extractor tension and some others that I'm forgetting all work together to make for a reliable controlled feed. JMB designed all this on a 5" slide. He also elegantly designed the 1911 so that each part tends to do more then one thing. So as you start changing specs, and getting farther away from the original you have to exponentially change more stuff. And the design window for all of those parts to function together gets smaller. Less then 4" 1911's are IMO too far down that slide. They can be reliable, but the window for tolerances is too small so it's harder to get them there, and harder to keep them there as parts wear. Also a 1911 with 8 rounds is at the bottom of what I consider acceptable capacity for CCW so dropping another round doesn't help. Lastly, as I mentioned in my post about commander's I have both a bigger and a smaller carry gun, so if I'm to the point in wardrobe or concealment need that I would be require an officer's 1911, I just drop down to the pocket .380.
So after that long rambling post that's why a 4.25" 1911 fits MY CCW needs. Obviously people differ so it might not fit yours. But if a hypothetical CCW'er has settled on a 1911, then I'd bet their requirements are similar to mine, just do to the kinds of thought process gun compromises that lead one to end up with a single stack, SA CCW. ANd this hypothetical person will probably, like me, find that a Commander is the goldilocks of 1911's. Not too big, not too small, but just right.
Hope that gives you an insight into one guy's thought process.
