I actually think that it isn't. The thing about terrorism is that it's a loser's fight. It's always a rebellion against a more powerful force. Even rebellions like the American Revolution and US Civil War were reactionary movements against change. Fearing an imposition on their rights, certain landowners rebelled.
The Middle East is Westernizing and Secularizing relatively fast. While many people might not think so, the difference between New York and Kabul, Damascus, or Kuwait City is a lot smaller than it was a century ago. Even Iran is a very Western nation, in clothing, architecture, and government organization. Nearly all terrorist sect in the Middle East has been formed to combat growing foreign (and usually American) influence. The main reason that Al-Qaeda hates the United States is because American soldiers were briefly stationed in Mecca and Medina.
ISIS is the most volatile and most flamboyant example of these terrorist groups, and directly exists as a way to combat the secularization of the Islamic world. Wahhabism is only popular these days because it's a rejection of Westernization, and an alternative choice to young Muslims who may feel pressured into acting like Americans. This makes its ideology popular with reactionary Muslims around the world. I have no doubt that the majority of ISIS members don't really want to live like seventh-century Bedouins, but ISIS is a group with power and a fighting chance that stands for many of the values these people support. For very conservative Muslims, ISIS is a chance to reverse what they see as a growing, horrific influence.
If Islam as a whole was actually going backward, a group like ISIS could never exist.