Probably because the chances of making such a bomb, while on board an aircraft, are incredibly low, even with the right equipment brought with you.
From the evil wikipedia -
'The explosive used was liquid nitroglycerin, which was disguised as a bottle of contact lens fluid. Other ingredients included glycerin, nitrate, sulfuric acid, and minute concentrations of nitrobenzene, silver azide (silver trinitride), and liquid acetone. The wires he used were hidden in the heel of his shoe. At that time, metal detectors used in airports did not go down far enough to detect anything there.'
That's just the ingredients.
Time, equipment and skill needed to construct this - in flight -, would be rare indeed, and more likely to kill the bomber, than do any other harm.
Any terrormuppets actually trying this would probably have more success by launching fireworks at aircraft engines as they take off..
If there are bombers out there with the skill to construct this type of device, these airport checks are not going to stop them... they'll just get a contact who is a baggage handler instead.