Oznayim la'Torah: Because, based on the Abarbanel, who explains that Yehudah (who Moshe compared to a lion) was placed in front of the camp - to confront the enemy, since in a war, the danger from the enemy is most acute in the front; whereas Dan (who Moshe also compared to a lion) was placed at the back to confront attackers, since the threat of attack by an enemy is most acute at the back, ¹ the Torah is teaching us that they encamped according to their flags - Dan, in their capacity of leader, in front and not at the back, ² as logic would dictate. ³