What is the primary reason a scientific theory is considered weakened when a hypothesis derived from it is disconfirmed in a systematic empirical study?

A psychologist proposes a theory about the role of sleep in memory consolidation and derives a specific hypothesis: participants who sleep for eight hours after studying will recall significantly more words than those who stay awake. A carefully controlled experiment finds no significant difference in recall between the two groups. Based on this single disconfirmed hypothesis, the theory must be completely abandoned because it has been proven false.