That entire airplane is one big fuel tank. When your ground level to an MD you can see how going out to the wing makes sense. Its wings are pretty low because they arent as load bearing like others in their class seeing that the power plants are on the fuselage. Get on the ground next to a 737 or A320 and your much higher. I believe the MD has 2 exits that send people to each wing and it would be the quickest way to get the passengers out quickly. And safely without everyone freaking and trying to get out those single doors and jumping higher.