1st, the USA is not the only subject of terrorist attacks. There are plenty of other nations that suffer at the hands of muslim extremists.
2nd, the USA squeels loudest when bit. Hollywood and the US media are easily manipulated by these organisations. For example, the PA vehemently try to control what footage is shot in that conflict. 6 months back footage was smuggled out that showed PA police engaging Israeli forces, despite the PA denying such things ever happened. The report took great risk in getting it out. The US media often tries to avoid falling into the sensationalism, but rarely does it avoid it. Its like the ultimate troll.
3rd, the USA symbolises a significant portion of what these people despise. Or at least the terrorists perceive it that way.
Nuthing like shoving a firecracker up the biggest kid on the blocks arse to get everyones attention