Why does the posterior segment of a severed earthworm generally perish?

It lacks the necessary vital organs

While earthworms exhibit remarkable regenerative capabilities, this ability is strongly dependent on which portion of the body sustains the injury. If the section containing the head and vital organs remains intact, that half can successfully regrow the lost tail segments. However, the popular belief that cutting a worm in half always yields two survivors is inaccurate. The posterior, or tail, section typically dies because it lacks the critical internal organs—such as the brain, pharynx, crop, and gizzard—that are essential for survival, nutrient processing, and coordinating bodily functions. Without these life-sustaining components, the tail section cannot sustain itself.