Because they are the main creators of it the past 40 years. The world's intelligence agencies can go on and on about lone nut patsies and Al-Qaeda but can never connect the larger dots for us? 9/11 was just a one off done by 19 Saudis bozos who various friendly intelligence groups were tracking but apparently uninterested in apprehending. The towers fell magically by getting smacked by planes and what seemed to be short burning fire. The Yemen guy is just some Al-Qaeda punk with explosives in his underwear. And Tim McVeigh is just a pissed off loner?
But you may notice how Tim McVeigh's wider network in the Neo-Nazi movement, Philippines, Germany and beyond are never explored by the intelligence agencies or the media? You may see how the media can't explain with a straight face how planes and a weak fire can bring two massive structures down on 9/11 when this has not occurred before? Notice how the Ft. Hood shooter is portrayed as just one lone Al-Qaeda nut with no inside aid who got passed around by U.S. officials although they knew he had been in contact with Al-Qaeda? And yet they moved him to Fort Hood anyways?
They have NO INTENTION OF EVER STOPPING TERRORISM. To stop terrorism you'd have to expose all of the "Inside Men" who keep allowing terror to occur on their watch and reveal to all what the mechanism is that allows this to keep happening(blackmail? Payola? a 5th Column?) If they stopped terrorism and shut down the terror networks which the western governments keep hidden from the wider public view then you would shut down the pork barrel spending on wars, defense contracts and weapons that keep them soaking up trillions of dollars.
Daniel 7 verse 19
19 "Then I desired to know the exact meaning of the fourth beast, which was different from all the others, exceedingly dreadful, with its teeth of iron and its claws of bronze, {and which} devoured, crushed, and trampled down the remainder with its feet,
You can't ask an unrestrained BEAST nicely to stop devouring everything in your kitchen. At some point you just have to force it to stop.