Because other forms of mass travel such as trains and buses, which barely have any security checks, rarely get blown up by bombs smuggled on to them, or hijacked by guns smuggled onto them.

That's because you can't really drive trains into buildings, and while you could technically do it with a bus, it's not that feasible nor that damaging. Flying a huge intercontinental airliner fully loaded with fuel into a building is quite different.

There's very good practical reasons the terrorists chose airplanes and not trains or buses.