I'd certainly describe the Pentagon as a symbolic target in exactly the same way as the WTC was a symbolic target. I don't fully disagree with the remainder of

I'd certainly describe the Pentagon as a symbolic target in exactly the same way as the WTC was a symbolic target. I don't fully disagree with the remainder of your comment, but I would also say that it's not a trivial factor that the targets you listed also have something else in common: they're all U.S. targets that are local or relatively easy to get to locations. Another consideration is that those are all older attacks. More recent attacks around the globe to Western facilities have focused on non-military targets. Based upon the result of 9/11 (U.S. engaging full force), the non-military target has demonstrated to be more productive for them.