Esco,
It's the size.
That's why something like the J-frame is so popular. The reliability of revolvers also figures into the equation, but with superb modern autos like Glocks and Sigs I really don't think reliability is a question in most people's minds, including mine; I have
almost as much confidence in those guns as I do in S&W wheelguns.
Believe me, even though my affection for small revolvers is deep-seated, I'm not so sentimental that I would give up the advantage of the auto's extra capacity just for the "cuteness" factor of the the revolver. I would love to be able to pack a Glock 26 or 27 instead of the J-frame. But these guns, while still small and with an amazing firepower-to-size ratio, are still much bigger.
Despite the cylinder of the revolver, which at its thickest point (diameter) is always more than an auto, the wheelgun simply takes up a lot less space overall because of how thin the frame is. That "thick" cylinder is only "thick" for a very small area. Otherwise, the gun is very svelte.
About the multiply BGs/single BG issue:
I'm surprised to see how many people questioned your original assertion about the prevalence of incidents involving multiple attackers. From what I have seen/heard and experienced, this is very true and I agree with you.
To my mind, there is really only one viable strategy for facing multiple adversaries with a small-capacity firearm, and it's the same approach that applies for a firearm of any capacity in such a situation: Force yourself to remain calm, identify the biggest threat and (provided the appropriate force "threshold" has been crossed, i.e., deadly force is warranted to protect life or limb) let the rest of the goon squad see what happens when a bullet at 1000 fps enters someone's skull or chest. Repeat as necessary, or until resources are expended.
Five rounds may not be many, but, wisely used, they will be sufficient to demonstrate to someone threatening your life that the stakes are high. If your attackers are inclined towards self-preservation, they will retire after a display of this sort, provided you have the time and composure to execute it.
If they are so numerous, suicidal or stupid that the termination of one of their own has no effect, then I guess your number is up. Please note that I'm not saying that I think a higher-capacity gun would be of little effect in such as situation. Quite the opposite; I'm pointing out that selecting a carry gun that is realistic in terms of its concealability involves playing the odds. It's often impossible to base your equipment selection on the worst-case scenario.
More rounds are always better. There's nothing comforting about limiting oneself to five rounds between reloads. For me to do this, there has to be a compelling reason, which there is - concealment.