There is something in the USA called the Constitution. In the Constitution there are certain amendments. One of them is the second amendment which says "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
It is very difficult to change this, especially since there is a very strong lobby in DC, led by the National Rifle Association (NRA) which supports everything in the second amendment except for the two words "well regulated". So any attempt to control guns in the USA will be vehemently opposed by the NRA and all the politicians who either support them or are too scared of making them angry.
Also, in order to change any article of the constitution, you need a 2/3 majority in both the Senate and House of Representatives. So its very difficult to get this law changed.