I think the postponement is clearly USAC's fault. Austin Parks & Rec has the right to protect their resources. USAC should have found a venue, or even just a course layout, that could handle a couple of thousand laps in the rain, and communicated this need clearly to the manager of the venue.
There should never have been a scenario where Austin's version of Leslie Knope could wake up on Sunday morning and realize the event wasn't going to work.