Because she'll be a "weight in the chain" and slow him down and risk him getting caught. because if he takes her, it'll signal that he's at the excavation site and it'll ruin the mission. because if he takes her, it means that she'll cause a distraction and compromise the Germans from finding the Ark. because he's so focused on the mission he wasn't bothered enough to distract himself with saving her. Why does Indy leave Marion tied up in Belloq's tent?