Because the "common" people are part of the problem.
Shit will never get fixed.
How are the common people "part of the problem"? That is a myopic perspective, which ignores the social factors which lead to the crime rates in the U.S and the mass shootings in the first place. Rather than put people in a straight-jacket, the goal should be to fix the fundamental problems which lead to people killing others: income inequality, poverty, drug criminalization, social atomization, capitalism, etc. I'd rather fix these more fundamental problems than treat symptoms.
And you really haven't addressed the costs of banning weapons. The murder rate, police brutality rates, productive resources, etc would increase in the medium term, and time and resources would be wasted in much the fashion that they were wasted during alcohol prohibition, and our current drug prohibition.
The U.S is too big of a country, with too many people to effectively ban guns without there being significant costs -- costs which exceed the costs of the social symptoms in the first place.
Ending the drug war, reforming the criminal justice system, and giving a universal basic income would be miles more effective at reducing overall murder rates than banning weapons, and it wouldn't put racial, ethnic minorities, undocumented immigrants, etc at the mercy of police.