Could it be the coticules ?
Kidding aside, we have a considerably larger population unless you're figuring per capita. Add to that our history. In the USA we won our freedom with guns. We defended ourselves and expanded our boundaries through wilderness areas with guns. From the pre revolutionary war days, through the Indian wars and the expansion that became the 'wild' west we have a history of rugged individualism and guns.
Europe suffered two brutal wars on their home soil. I suspect that was enough violence to last many generations.