For any conspiracy, it is worth thinking about the planning session right at the start. Ask the questions what were the aims and of the literally thousands of possibilities, does this stand out as the safest/lost effective/least risk ... In short, the front runner. Now it seems to me that the twin towers from the terrorist view was low risk in terms of detection and set-up, since relatively little needed to take place on US soil. However, back then, there were lots of other possible targets where the effect was more predictable. The destruction of the Lincoln Memorial for example, would strike at a major symbol of what in means to be American in a way that a couple of banking offices might not. I'm not at all convinced that someone unconstrained by the risk of detection would have made the same choice