I'm certainly no expert on anything but beer drinking and fajita eating, but...
1.The flannel-heads considered the twin towers prime targets and attacked them previously.
2. The forward speed combined with the mass of the aircraft would produce enough energy through the heat of compression at the points of impact to slice through the towers perimeter load bearing members like butter.
3. Add the heat from the jet fuel to a structure with diminished load capacity and a severely damaged fire suppression system and it's probably a testament to the engineering that they stood as long as they did after being struck.
4. And fuck a buncha truthers.