1.The quest for power: since the beginning of time men have used polarizing arguments to motivate the masses in order to achieve their own personal agenda’s.
2.Propaganda/brainwashing: we all start getting brainwashed the day we are born. Our parents indoctrinate us in their core beliefs and many/most of retain some or all of those beliefs for our entire lives. It’s not malicious but it’s certainly a form a brainwashing and it’s part of the human condition. The cultural environment you’re born into has a lot to do with who you’re going to become. If you were born elsewhere and/or raised by different parents you probably would be a very different person.
3.Radicalization: This is where someone typically for personal power reasons misuses the religious/cultural message and brainwashing people have already received. It’s more vehement, it’s more invasive and usually pitched in an immersive nature. This is how cults “brainwash” their members. Full immersion in a new culture with a radical message and usually coupled with isolation from outside intervention or different views. It basically is the advanced use of point 2 in the pursuit of point 1.
4.Poverty: This is not critical but it makes people more susceptible to brain washing. The worse off your current predicament the more likely you’re receptive to a radical message, particularly with a promise of something better as the carrot. Thus the 72 virgins at the end of the rainbow. It’s a lot harder (but not impossible) to talk a 17 year old with an x-box, a car, money in his pocket and a girlfriend into strapping on a bomb vest than it is a kid with little or no education, no economic prospects and no shoes on his feet.
Human nature: There are so many aspects of the human psyche that allow us to be manipulated by charismatic leaders. The need to belong, the quest for a cause and the sense of purpose that entails. The promise of something better or the mob mentality created by “us vs them” propaganda. You could write a book on all the social and psychological factors that radical leaders exploit when creating a following. They determine what you’re looking for and then they promise it to you if you follow their agenda. This works equally well in individuals or groups though the methodologies are slightly different.

Ultimately Islam is just the tool being used to frame the message by a lot of terrorist groups because it’s already indoctrinated in most of their target audience in the places they are growing their movements. The fact that Islam is the religion in so many poverty stricken locations and the core message so easily twisted to violence only exacerbates the problem. As with most ancient religious texts it’s pretty easy to spin whatever message you want out of the convoluted messages being broadcast more than 1000 years ago.