I don't know the reason. However, last summer when we went with our family of 5 they let us stay in the Mansions. I asked, because my son was too big for the trundle bed and he wasn't willing to sleep with one of his sisters 4 and 6. So we brought an air mattress for him and I asked since we wouldn't be using the trundle bed, could we please stay in the mansions buildings. The CM went to check and sure enough since we had our own air mattress it was ok. I really staying there.
This year we will only be at POR for a couple of nights so I am going to tell my son he is just going to have to deal with sharing with one of his sisters for a couple of nights.
The CM didn't allow you to stay in the mansions because you said you didn't need the trundle bed and had an air mattress.
Most likely the AB section was overbooked when you got there. The fire marshal will allow a very, very, very small number of rooms in the mansions to hold a 5th if needed. This will not effect the overall exiting in the case of a fire.
If they had to do more than one, they would most likely scatter them around the complex, in order to not violate the fire code.
But as I stated previously, only the buildings in AB could accommodate 5 in a room, if ALL the rooms were occupied by 5 people (the Fire Marshal considers worst case scenario), the aisleways and stairways at AB could handle the load in case of a fire.
The mansions could not physically exit everyone if every single room had 5 people. You have to remember that elevators go down in case of a fire and cannot be considered fire exits, because they are shafts and carry a fire upward through the building. So they go down automatically. When you look at the size of the buildings in the Mansions, the aisleways and stairs could not physically exit that amount of people in time in case of fire (again, considering worst case scenario), if ALL the rooms were occupied by 5 people.
In the case of fire exiting, you always have to consider the worst case.