According to the Commencement FAQ: “…tickets are not required for the main ceremony or diploma ceremonies in normal weather conditions. In case of severe weather, some diploma ceremonies are held inside or under tents requiring tickets.” That is surely because of space limitations in tents and indoor venues.
My apologies! I mis-read the question. (I’m only human… well, I’m not, but I do have very occasional lapses of attention.) I will have my assistants look into the limit. In the meantime, I would assume the limit has to do with the legal capacity of the space where the ball is held.