Supposedly, because you need the Guinness World Records' seal of approval for something like this. But, from the sound of it, that will never happen.
According to Mainichi News, when the city submitted its outdoor toilet, the Guinness folks replied by saying that not only were they uninterested, but they would not judge the submission. That's right, the "world's largest toilet" was rejected, perhaps, because this submission was just too crappy?
Ishihara said it's not giving up on promoting its outdoor toilet as a tourist spot. But how long can they continue? It costs US$125,000 a year to care for this bathroom. At this point, that's like flushing money down the toilet.