Roboworld is a dangerous place for the racer. When they announced that the race was to be held in Roboworld, the President of Roboworld directly stated that they would not allow the race to be held there, and they would destroy any outsider who entered their territory by any means necessary. They know that Roboworld had many troops and weapons to stop the race. One of the racer even stated that Roboworld troops were the meanest troops in the galaxy. So why did they still decide to race there? Was there a reason behind it or did they just want an intense race?
Being attacked by the local military made the race just that much more exciting.
The racing culture seemed very counter-culture or anti-authority as well, so holding the race somewhere they were forbidden from just jived that much more with their ideals.
Anyhow, there was a nearby refugee moon where they were basing out of (if I recall correctly) so it wasn't too hard to pull off logistically... especially considering some of the race organizers literally had magic on their side.
But to answer your question, I don't think there was any single, specific, reason.