(First of all, why is this on the LoL boards?)
Second, it's mostly a cultural thing, and a very controversial one at that. Given that we're a state formed by armed revolution, it's built right into our constitution. The gun lobby and the National Rifle Association are fairly strong political forces in America, and since we only have two major political parties, the party they gravitate towards can't afford to offend them or they'd lose a significant portion of their voters. Since the two parties are fairly evenly matched, progress in any direction is very, very difficult to achieve.
A lot of it is tied to a 'cowboy mentality', where people see themselves as the first line of defense against crime, and guns as a way to protect themselves and their families. A lot of this is cultural leftovers from older eras, which are very hard to shift with anything except time. It would take a tragedy on an epic scale (far larger than the daily mass shootings we experience; there were no less than five on the 11th) in order to enact large-scale immediate change.