The colosseum said this was to discourage cheating by going off track and taking a short cut, but I think the real reason was they liked the spectacle of it. People didn't fall off often, but when they did, it made a big splash, for lack of a better way of saying that.
Technically, there wasn't any rule against taking a shortcut. This was simply because trying to do so would be the equivalent of signing your own death warrant. No one had ever tried it before and lived, so no one tried it anymore.
That was until Mira came along. When the three other competitors came around the corner, I stood up. I don't really know why. Maybe it was out of nerves, or fear, or maybe it was just because I wanted to be able to see better.