Some might say religion.  Some might say Godless societies.  Others might just say Islam, oil, or George Bush, and be done with it.  I vote for ignorance and illiteracy:

In Saudi Arabia I once met two young members of the National Guard, Saleh and Abdullah. As we drank coffee in a cafe, I asked if they knew anything about Jews and Christians. They didn't, but Abdullah seemed curious. He said he'd like to read a book about other faiths, but none was available in the kingdom.

Saleh objected. "Well, what if you read a story and it made sense to you so you believe it," he said. "It can reduce your faith."

They both agreed: Better not to read.

Any society where knowledge is filtered by controlling speech through a higher figure, whether it be one in government or any religion, is doomed to extremism and violence.