Is there any reason why the race takes place on Roboworld?
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.