Because the WTC building has security, and hijacking a plane and crashing it into a building easily fits the pattern and MO of al Qaeda attacks. Suicide bombings and plane hijackings are both common to the organization, combining them isn't such a leap. I saw an interview in Thomas L. Friedman's (unfortunately) short lived documentary series where a French Muslim woman described using America's own technology to destroy it's buildings as "A well deserved slap in the face" to America. It's symbolically more effective. Furthermore, they could carry out a lot more destruction. I think someone would notice if they tried to demolish the building with explosives.