It seems there's a lot of money to be made selling firearms and ammunition. And money talks when it comes to lobbying politicians.
Also, the USA is known to be one of the most capitalistic countries in the world. The ability to make as much money as possible without encountering too much red tape is one of the country's highest priorities. They tend more towards the side of eschewing regulations in favour of the free market, not just on firearms but on pretty much anything. That's why it's the world's largest economy and one of the best places to do business, but also why they have few workers' rights, very little socialised healthcare, an obesity epidemic, high carbon emissions, and indeed, a gun control problem.