There might have to be some clarification with the question. The idea of "certain areas" might not be as valid as one might think. There is such a difficulty in defining terrorism as a concept because it is so pervasive and something that impacts every nation. As long as there is disenfranchisement, there exists the capacity for terrorism. This is not to say that disenfranchisement is terrorism. Yet, rather to explain that all areas of the world have the capacity for terrorism. There will be individuals who recognize the opportunity to purport terrorism as a way to achieve their goals. In a broad definition of terrorism, mob action or riots can be seen as terrorist activity. Hate crimes perpetrated against particular individuals can be seen as terrorist activity. This is not something limited to "this area" of the world and "that area" of the world. It should be noted that not all terrorist activity all over the world is equally covered and represented by media outlets. "Certain areas" might receive more coverage to fulfill bias or to perhaps further the idea that terrorism is not fluid or amorphous, but rather static, localized in a few areas of the world. I don't see this as true. In a globalized world with increasing technologically efficient means of communication and networking, terrorism is fluid has its reach in all aspects of social interaction all over the world. If there is a commitment to seek to eliminate terrorism, then I believe that there has to be discussion about how it is a universal reality of the modern setting. I don't fully subscribe to the idea that it is "heavier" in one area over another. I think that prior to September 11, few in America would have seen terrorism as something real and heavy in America. Yet, we now do. Interestingly enough, domestic terrorism exists everywhere and I would suggest that its presence is as heavy as anything else. I think that the presence of domestic terrorism in nations makes terrorism present in all nations and not merely localized or concentrated in one area over another.
Terroristic activity is often concentrated in certain areas of the world, since those areas most likely have ethnic groups which do not get along with each other (ex: the Middle East). Areas of the Middle East are holy lands for Jews, Christians, and Muslims alike. Although they all worship the same god, but with different names and some different ideals, they don't get along because of misunderstandings. Eventually, religious fundamentalist extremists get into the mix, and there we have it: terrorism.
Also, since many western countries have oil interests in the Middle East, they tend to get involved, causing even more extremists, who claim to have the duty of purging all imperialists, infidels, or those who challenge THEIR interests, and there we have terrorism escalatin to international levels.