Why is the cause of late Pleistocene megafaunal extinctions still debated?

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Map showing the extent of the Laurentide, Cordilleran, and Greenland ice sheets during the Wisconsin glaciation, roughly 100,000 to 11,000 years ago. The Laurentide Ice Sheet covers most of present-day Canada and parts of the northern United States in light blue, the Cordilleran Ice Sheet covers the Pacific Northwest in teal, and the Greenland Ice Sheet is marked in purple. Modern international and state or provincial boundaries are outlined for reference. — Pleistocene Epoch Ice sheets during the Wisconsin glacial episode penetrated deep into North America.

The cause of late Pleistocene megafaunal extinctions remains a subject of vigorous debate because the two primary hypotheses—human overpredation and abrupt climatic change—coincide in timing, making it difficult to isolate a single driver.1

The Competing Hypotheses

The Overkill Hypothesis: This theory suggests that skilled human hunters, such as those associated with the Clovis complex, decimated populations of large herbivores that were unaccustomed to human predators. The subsequent decline of these herbivores would have also caused the extinction of large carnivores.1
Climatic-Change Hypothesis: This perspective argues that the transition from the last glacial period to the interglacial period caused severe environmental stress. This included the reorganization of vegetation, changes in food availability and nutrient value, and increased seasonality, all of which disrupted ecosystems.1

Because both human migration and significant climatic change occurred during the same window, researchers increasingly suggest that these factors likely acted in combination. It is probable that the relative importance of human activity versus environmental shifts varied significantly depending on the geographic region and local conditions.1