The area between Buchuma Gate and Aruba is basically savannah - flat plains with quite long grass and occasional bushes. This provides little cover from predators or protection from the sun. Consequently there are few animals to 'spot' and so the ground is covered quickly.
Coke's Hartebeest are quite common in Tsavo East but the Hirola is a very similar looking animal which has longer straight horns, but still with the hump on the back. It is much rarer & we've yet to see one.