Why hasn't America been attacked since 9/11? America is being attacked every day. Almost every terrorist attack is in effect an attack on America, every soldier shot at in Iraq and even the bombs in Madrid. It an attack on the on what America represents. The freedom we enjoy in the west is seen as 'American' in character. It's symbolic as much as anything. America is not simply a place it's represents a way of living which the extremists hate. So 'America' is always under attack.
But as to why the 'Homeland' hasn't attacked. Well, if you actually look at terrorist attacks attributed to Al Qaeda in places like Madrid, Bali, Turkey or attempted attacks as in Britain. The one common factor is the they are carried out by local extremists usually with help from the outside. But basically the terrorists were either citizens of the country concerned or immigrants recruited by the extremists. So by extrapolation the most likely source of terrorism on the American continent will likely come from US Islamic extremists. American citizens born in the USA. Free as America is I doubt if extremists find it easy to go unnoticed in the US these days.
But they exist I'm sure. Like the American convert to Islam whose fingerprints were found on parts of the Madrid bombs.
On the other hand Al Qaeda has been badly damaged by US action particularly in Afghanistan. Whatever you say about the war in Iraq the one in Afghanistan has been relatively successful.
I'm sure someone is planning something but I'm equally sure the FBI is on the case.