It'd take a significantly larger weapon to do that, due to the area covered by most airports. With the plane, they're guaranteed a certain number, in the airport, they might not actually kill anyone (Which would be preferable for those of us not really wanting to be exploded). It's pretty grim logic - But it's undeniable. That, and people around the world are more likely to react to a plane being exploded/crashed into something. Which would 'further their cause' or whatever the hell they do it for.