1. To kill a large number of Muslim extremists hiding in difficult terrain very dead very quickly.
2. To make a large noise and commotion capable of shaking up a few North Korean savages very far away from that (first) MOAB bomb strike.
3. Because it is a cool weapon that Bush and (of course) Obola were scared of deploying, because it might upset the Muslim extremists in their national press corpse.
Robert A Cook gave the best answers.