They know exactly how many people are up there and I presume can fit them all in one lift. With the WSU they'll have no clue of how many people could need assistance.
I actually emailed PB about this when my wife was pregnant and struggling with the stairs - I asked if we could use the lifts as a temporary measure for a couple of games. His reply was that they don't want someone who can't use the stairs stuck in WSU should there be a need to evacuate, the...