The military industrial complex works great when there is an enemy to fight that requires the production of military equipment. examples of this enemy, in the past, were the Axis powers and the USSR during WWII and the cold war. It doesn't work so well when there is no enemy to fight.
after the dissolution of the Soviet Union in the early 90s, there was an "enemy vacuum": who would be the next enemy that the US could use to drive the military industrial complex? Russia wasn't worth fighting indirectly any more because they were too weak, and there really wasn't a competitor that was strong enough or threatening enough for the US to parade around as the enemy.
enter terrorism: it provides a target that fulfills a number of very crucial aspects:
1) non state actors: the US can go to war with terrorists without going to war against the officially recognized government. This is key to global security because conducting war within a nation's borders no longer constitutes an assault on that nation. This prevents WWI type wars that start as a series of cascading mutual protection treaties or similar.
2) difficult to define the enemy: who are terrorists? whoever the ruling party deems terrorists. everyone else is a freedom fighter. This makes it convenient to come up with a new enemy group of terrorists when the current batch has been "defeated" or the american public becomes war weary.
3)easy for the government to drum up support for new military action by creating and appealing to a sense of fear within the populace and a desire for "safety".
4) oil: the majority of terrorist groups that are in the focus of the american public's attention are primarily located within countries that have large oil interests. Other terrorist groups located in other areas, such as the philippines or south america, are largely ignored.
5) percieved threat of terrorists: it is easy to believe that terrorists are a significant threat to US citizens precisely because of their ability to strike on american soil. This greatly adds to their value as a enemy for the military industrial compelex because the general us population sees the threat as legitimate. (a converse example of this would be if the new enemy would have been, say, brazil: they're not really a threat to the US because they wouldn't really be able to directly threaten the american mainland.)
there are other reasons, i'm sure.