Why do some classifications place the hoatzin in its own order, Opisthocomiformes?

The hoatzin is sometimes placed in its own order, Opisthocomiformes, due to significant anatomical differences that distinguish it from other birds, particularly those in the order Cuculiformes.1

While some classifications group the hoatzin with cuckoos, its foot morphology is distinct: hoatzins possess three toes directed forward and one directed backward, whereas cuckoos have a zygodactyl arrangement with two toes forward and two behind. Furthermore, the hoatzin possesses unique internal features and a specialized digestive system that ferments vegetation, a trait not found in other birds. These structural and physiological deviations from the standard characteristics of the order Cuculiformes have led many scientists to categorize the species within the separate order Opisthocomiformes.1