there's also the fact that it's easier to blow up a plane. you need a big bomb to destroy a bus but only a small bomb to make a hole in the plane and kill everyone. and the plane has way more people on it than a bus.
A hole in a plane's fuselage or wing doesn't mean that everyone dies. You need a lot of power in order to take out a wing. Remember that wings on commercial jetliners are designed to eat an entire disintegrating jet engine in order to protect passengers in the fuselage.
Additionally, a jetliner's fuselage can take a VERY large hit at cruising altitudes and speeds [0] and still leave the plane flightworthy.
Planes are tougher (and their crews better prepared) than people think they are.
