Ok. How about "because the states stopping them were sabotaged by external actors". In Iraq's case pretty obvious. From a local's perspective, here's what the US did :
1) remove nationalists from power by destroying all infrastructure. They sucked, but they sucked for everybody and they weren't nearly as bad as religous muslims.
2) jobs ... gone. Money ... gone (or gone soon). Family ... partially gone. Tribe ... partially gone. Security forces replaced by US troops, who don't speak the language.
3) this situation persists for years.
4) (for Sunni's, Christians, ...) US replaces security forces (suddenly) with an army exclusively formed by a religious grouping that Sunni's are attacking wherever they can (Bahrain, Yemen, Saudi Arabia, Egypt, Afghanistan ...) who are looking for revenge.
5) These new security forces come in, US forces don't keep them in check, then suddenly abandon everyone. This sort of stuff becomes commonplace : http://www.bloombergview.com/articles/2015-03-11/shiite-mili...
6) several tens of thousands of people that this happened to were soldiers.
So why is islamic extremism spreading ? For Iraq, you have the answer above this sentence. For Europe/America the answer is different. It is not spreading within the older generation, people who fled the middle east. It is spreading within the youth. How ? Well :
1) schools, bankrolled from Saudi Arabia and Turkey teach islamic supremacism to the kids of immigrant muslims.
2) western european states do token cleanups of these schools every 10 years or so, but not fast enough to make them stop.
3) these kids are not educated to the level even bad local schools educate local youngsters. At 18, they are suddenly abandoned, and quickly find out that no job, no family (not possible without job), no car, no status symbols, ... sucks
4) furthermore. Suddenly islamic supremacism educated kids finds society (including muslims) shun them, or worse. Why ? They have no decent skills, and because of the supremacism thing they are prone to violence.
5) they subsist on welfare for a while, generally causing trouble around them and little else.
6) someone offers them a purpose. "Free" brides (ie. kidnapped women and children. Like most muslim scholars, daesh sees nothing wrong with forced marriage at 6, after all the prophet did it. Makes you wonder how "normal" muslims morally deal with that). A state and police force that supports the supremacism and a free ride over. Oh and a monthly paycheck that more or less matches local welfare checks and what you might call life insurance.
The biggest thing stopping them: daesh has a reputation for getting people killed and abandoning them.
Let's get real here : if the US did the same to a Western European country it did to Iraq, there'd be isis/daesh territory in the middle of Western Europe, no doubt.
Also let's face facts here, and scale the problem. Daesh/islam will not be recruiting really large amounts of muslims from the western world. But it is growing : these people will cause a civil war in Europe unless the situation changes.