Kidneywood trees (Eysenhardtia texana) are small trees species that prefer to grow in full sun to light shade. Due to their palatability, this makes them highly susceptible to being over-browsed by deer, and, in some cases need to be protected by tree fencing. It's possible that conditions along the trail became too shady for the species. This is common in the successional process and may be a sign that this area of the park (Oak Loop) is mature with large trees species.
The kidneywoods around the PHP UEC are exposed to more sun and thus are in conditions more suitable for growth of this tree. Where necessary and where conditions are optimal for these trees, small specimens can be fenced for added protection, but locations along Oak Loop Trail may not be ideal.