The most likely reason is that the switch port is misconfigured. The scenario states that the switch port is configured for half duplex. In modern Ethernet environments, especially when connecting through fiber media converters or higher-speed switch links, duplex and speed settings must be compatible between devices. A mismatch or unsupported duplex setting can prevent the link from establishing correctly or cause serious connectivity problems. Since the cables have already been successfully tested, the connectors are seated properly, and the media converter is powered on, the configuration of the switch port becomes the most likely cause.

The CompTIA Network+ N10-009 objectives include troubleshooting physical connectivity, interface configuration, speed and duplex mismatch, and media converter issues. A transposed TX/RX fiber pair can also cause a no-link condition, but the question provides a specific suspicious configuration detail: the switch port is set to half duplex. One of the cables having a short is unlikely because both cables tested successfully. Interface errors usually occur after a link is established and traffic is passing; they would not be the best explanation for no link light. Therefore, the switch port misconfiguration is the best answer.