The National Rifle Association is perhaps the most powerful lobbying group in the USA and no politician can ignore it's influence and know it would be electoral suicide in many areas to get on the wrong side of the organisation. They are heavily funded by arms manufacturers and they use their money and power to win over prominent politicians to their cause. There are many people who are so blinkered by their love of guns and their hatred of all things liberal they even claim that the tragedy didn't happen and it was an elaborate hoax to convince people to demand more effective gun licencing laws. Meanwhile every such incident results in an even greater uptake of firearms by the ordinary Americans.