In the UK, it was the Dunblane incident that triggered this (you can google it). In the 1990s, some madman with gun just strode inside a high-school at Dunblane in Scotland and started indiscriminate firing. Many teachers and students were killed and after this event, the UK government just outlawed the guns in the hands of civilians.
Maybe, in USA such an extreme event hasn't happened yet, so they have not banned guns yet.
--edit--
Corrected period.