> we're not dumb enough to allow rampant legal gun ownership
I respectfully submit that much of Europe's "enlightenment" in this area (including the UK) is not due to the wisdom and foresight of its leaders, but due to the historical accident that it was already densely populated when firearms were invented.
Widespread gun ownership was a reasonable decision in the US at the colonies' inception, and once it's implemented, the switching costs of changing to restricted ownership permanently become high. I'll elaborate on this:
When the US colonies first started, they had a tiny population trying to hold their own, spread out in a vast wilderness. A gun can be used to defend against criminals, hostile natives, or can be used for hunting. Remember, homes were often miles apart, there were no telephones to dial 911, and the nearest police force might be in one of relatively few cities or forts, tens or hundreds of miles away. Not letting people own weapons would have made the colonists' lives much harder.
Once enough guns are out there, it gets harder and harder to argue that they should be outlawed, because:
(1) A culture of gun ownership develops, which will be upset if you try to take them away
(2) A sizable number of guns will presumably escape the confiscation program and enter the hands of criminals
Add to this, the country's experiences during the war of independence convinced its founders of the benefits of empowering the citizens to rise up against foreign invaders or an oppressive government.
As for why there's not more support for "common-sense regulation," no less a hacker than Eric S. Raymond has recently given a cogent explanation [1] [2].
[1] http://esr.ibiblio.org/?p=4912
[2] http://esr.ibiblio.org/?p=4939