Why is it so difficult to determine where terrorists will strike next?

This is a good question, because prevention of terrorist attacks is determined almost entirely by knowing when and where they will happen. In his book The Black Swan, Nassim Nicholas Taleb uses the terrorist attacks of September 11, 2001, to illustrate the importance of what he has termed a “Black Swan” event. A Black Swan is an event with three attributes. Quoting Taleb:

First, it is an outlier, as it lies outside the realm of regular expectations, because nothing in the past can convincingly point to its possibility. Second, it carries an extreme impact. Third, in spite of its outlier status, human nature makes us concoct explanations for its occurrence after the fact, making it explainable and predictable.

Taleb goes on to use September 11 as an example of what you don't know being more important than what you do know. He claims that if the September 11 events had been conceivable on September 10, measures would have been taken to prevent the attacks. In this case, the events happened precisely because they were unexpected. Therefore, knowing New York is an easy terrorist target becomes inconsequential if the enemy knows you know it. In other words, if we had been convinced that New York was an easy target for terrorists, we would have put protections in place. If the terrorists knew we had these protections in place, they would have carried out a different attack—one we hadn't predicted and, therefore, not prepared for.Taleb goes on to emphasize his point that “we tend to learn the precise, not the general” by illustrating what we did and did not learn from the September 11 attacks. Taleb says,

What did people learn from the 9/11 episode? Did they learn that some events, owing to their dynamics, stand largely outside the realm of the predictable? No. Did they learn the built-in defect of conventional wisdom? No. What did they figure out? They learned precise rules for avoiding Islamic prototerrorists and tall buildings.

We have not learned what to do to avoid terror attacks in general but have put measures in place to keep these same attacks from happening again. But why is it so hard to put general preventative measures in place?The crux of Taleb’s argument in The Black Swan is that we live in an increasingly complex society where minor events rapidly snowball into the causes of larger events. As information flows increasingly rapidly through our society, these events become harder and harder to predict. At the same time, these unexpected “Black Swan” events become increasingly impactful to society. Taleb argues that we are incapable of predicting these extremely impactful events but refuse to acknowledge our limitations.There is an additional factor at play when it comes to preventative measures. Taleb puts forth a thought experiment where a legislator is able to enact a law that imposes continuously locked bulletproof doors on every airplane cockpit and, therefore, unknowingly prevents the September 11 attacks.This legislator is not touted as a hero. With the attacks having never happened, his actions seem unwarranted. It appears that he is simply squandering resources and may very well be voted out of office. He retires with a great sense of failure, feeling he has accomplished nothing.Taleb points out that it is not those who take preventative actions that get rewarded, but those who manage the aftermath of a disaster. Taleb asks,

Who is more valuable, the politician who avoids a war or the one who starts a new one (and is lucky enough to win)?

In short, while it may seem intuitive that predicting terrorist attacks is extremely complex, we have a hard time accepting that highly impactful events are entirely beyond the scope of our ability to predict. This limitation, coupled with our failure to be able to recognize the value in preventive measures, not only makes these types of events possible, it makes them increasingly more likely to occur.
https://www.nytimes.com/2007/04/22/books/chapters/0422-1st-tale.html