+Jeff: Norway's had 1. How many have we had in the US? Our regulations do not adequately allow us to keep weapons out of the hands of those with mental issues who should not posess them. So yes, it is in fact an issue of regulation.
Add on the fact that the regulations we do have, the gun lobby has managed to have made effectively impotent, as they have blocked hiring of BATF agents, made laws which actively prohibit their enforcement, made inspections of dealers "optional, and unrequired", etc. So yes, it's an issue of regulation.
As for the "defense against the government" argument, the main problem there is that the technology of the time allowed an armed populace to be armed at almost the same level as the government, and no one armed man could easily kill large numbers of people with their weapon - they had muskets and swords. Now the private citizens have weapons which can easily take down a crowd, and the weapons and training of law enforcement and the military have far outpaced the ability of the populace to defend against - except it's likely that any government lockdown would likely result in the government fighting against large chunks of the military itself. Having a volunteer military gives us the check against the government we need.
We need to restore the ability to enforce our existing regulations, update our regulations to properly detect and keep guns out of the hands of unstable people, and yes, improve mental health care for all those in need of it - not just those who have been deemed too unstable to own a gun. Our mental health system is atrocious.