Law enforcement and intelligence agents are better able to track terrorists than they were before 9/11 primarily because of better technology; but more importantly, because of the sheer number of people dedicated to the task of gaining clear and useful intelligence. It's really a matter of simple logic and numbers.
Law enforcement and intelligence agents are better at tracking terrorists than prior to 9/11. Technology has gotten much better, and officials are more prepared for an attack, or chasing down leads once it occurs. Governments have also been cooperating more. The Turkish police arrested the nightclub attacker after just a couple of weeks.
Technology is advancing, faster and faster are new methods of doing things being developed. We have new methods of tracking, new intelligence strategies help us better deal with any incoming threats. Not only are there new things that we can utilize, but we also know more about these terrorist organizations. We know how they fight, what they do, and what their objectives are, but most importantly, how we target them.
Intelligence agencies and their practices keep getting stronger. We are getting smarter and smarter all the time, making it much harder for a person to commit a serious crime without getting caught. I do believe that 9/11 may have played a small role, but I think the intelligence agencies would have grown stronger regardless, eventually.