According to an article in USA Today, there have been 221 mass killings (killing of 4 or more people, not counting the killer) in the United States just since 2006, less than 7 years ago. That's about one mass killing every two weeks. That fact alone should spur our government into taking action, without even considering the many thousands of other gun deaths in this country every year.
Why is this happening? The first reason can be seen in the graphic above from Mother Jones. We have too many guns floating around in our civil population -- nearly one gun for every citizen in this country (man, woman, or child). The second reason is that these millions of guns are easily accessible by anyone, including criminals and other dangerous people who should not have a gun.
But the third reason is perhaps the most important -- too many of our politicians want it this way. After the Sandy Hook school shooting a few months back (where several 6 year olds & their teachers were shot), about 90% of the American people demanded that Congress plug the holes in the background check law (holes that allow anyone, criminal or otherwise, to buy a gun without being checked from an individual, a gun show, or off the internet). But the politicians (Republicans and blue dog Democrats) refused to pass that law.
Those politicians decided that allowing everyone in America, including criminals and other dangerous individuals, to easily get a gun was more important than protecting the lives of innocent American citizens. They will give other reasons for killing that bill, chief among them the Second Amendment. But this is not a Second Amendment issue at all.
The Supreme Court has already ruled that the Second Amendment does not apply to all citizens (and criminals and the dangerously mentally ill could be prohibited from buying or owning a firearm). Whether they want to admit it or not, their vote was NOT to protect the Second Amendment -- it was to allow criminals and other dangerous people to easily purchase a gun (and use that gun to kill others).
This is not going to change. The mass killings will continue to happen, probably at an increasing rate. And it won't be stopped, or even slowed, until we elect some politicians that think innocent lives are more important than allowing criminals to have guns. And it looks like that could be a while, if ever.