By their very nature, terrorists mainly strike where they stand to strike the most terror into the hearts of their targets.

Boats and cars are limited in their strike capability. A fully fueled commercial aircraft is an incredible weapon with an endless list of targets.

It's just not enough to blow them up on their own any longer. Why take out 200 in the air when you can add the destruction of a politically symbolic target and the deaths of others to the carnage in order to send a bigger message?

I'd also like to add that talk of security measures is pretty stupid if we don't discuss WHY Islamic fundamentalists want to harm us. Once you figure that out and work to mitigate it, the security part is minor.