Like all of the enclosures at this open range zoo the white rhinos have a lot of space to move around, but arguably this is one of the smallest of the exhibits. Many of the hoofstock paddocks are multiple acres in size!
A possibility is that the cost of fencing that prevents rhinos from escaping is very expensive. While they are updating the fence now because of the rhino that escaped I think they are going to make it a bit bigger.
On the southern side of the zoo (which I didn't get to visit this trip), they have a large area for Black Rhino, which I seem to remember used steep banks to keep them in ? Can't remember in detail.