The aim of the WTC attack was to make a very symbolic action: the goal was to destroy three symbols of American power, its military, economic, and political might, though the last ended up physically unscathed. Obviously a huge number of deaths associated with it, but the symbolism was key. You knew exactly where the attackers stood--American power is evil--once you heard a description of the attacks.

This, in contrast, is incomprehensible violence. I can think of a dozen different viewpoints that could have driven it, but we'll have to wait for some idiotic statement from the criminals who did it before we have a clue "why" they did it.