Because the United States has a serious problem with violent inner cities, ghettoes, fatherless black children, and the culture of violence in that sub-culture.
Step outside those urban centres and the rate drops radically. For instance, Idaho has the highest number of gun owners, 56.4% of adults own guns. Murder rate 2015: 1.9 per 100,000. Vermont has the loosest gun laws in the USA. Murder rate 2015: 1.6 per 100,000. For comparison, Canada's murder rate is 1.68 per 100,000 in 2014.
The murder rate in the USA is now rising, but was at the lowest point in 60 years in 2015.........despite record gun sales, and the continuing liberalization of gun laws.
One of the safest democracies in the world is Switzerland, with a murder rate of 0.69 per 100,000. Want to talk about guns in Switzerland? Didn't think so.
I think that answers your question.