He deserves such an award because he played vital roles that made Argentina win the world cup. He didn't allow all the efforts of Argentina players to go in vain just like the goalkeeper of France who was not able to catch up most of the game. Martinez Emiliano deserved to be the best goalkeeper of the world cup.
There were other goalkeeper too who did so well during the world cup - although the players played so well. and at the last moment the whole burden in on the goalkeeper. luckily Martinez won.
Emiliano Martinez has played one of the roles behind Argentina's victory. He made a great save in the last minute of extra time. In view of which the game went to penalty shootout. And finally Argentina secured their dream final victory.