You need to read up on OBL and islamist ideology. He attacked the US because it had troops in SA to defend against Saddam Hussein. They were infidels (and some even women, gasp!), and not allowed to "defile the holy land with the feet of kufars". He submitted a plan to defend SA with his fighters from Afghanistan, but was snubbed by the SA government. For a narcissist, that's an unacceptable blow to the ego.
His choice of WTC was because he regarded it as an idol. According to his reasoning, westerners stopped worshipping as christians and worship material goods (this is Marxist thought, look up "commodity fetishism). Thus, they are apostates and no longer qualify as People of the Book, who should be respected according to islam. So they're fair game, as are their "idols".