Besides the fact that the battle between the Pandavas and the Kauravas was already fought, decided and over before it even began, as revealed by Krishna in his visvarupa to Arjuna, the reason the Kauravas lost was basically due to an act of apparent stupidity which can be traced back to one pivotal incident.
Krishna was taking an afternoon nap when both Duryodhana and Arjuna came to his place. Duryodhana came a little earlier and, being a proud person, felt his place should be at the head of the sleeping Krishna. Arjuna, who came a few moments later thought his rightful place should be near the feet. So when Krishna awoke and sat up he saw Arjuna first and let him have the first say.