Instead of thinking of the problem as "how to get the group of people to sit down the fastest", the problem should be thought of as "how to get the group of people to put their bags away the fastest" (or at the same time).

From the simulation it looks like the bottleneck forms because each person takes a minute to put their bag away. So if you have 1 person in the last isle putting their bag away they are basically blocking the entire plane.

With random you get random people who can access the overhead compartments all over the plane which helps increase the number of people simultaneously putting bags away.