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Why did the SCR-270 radar warning at Pearl Harbor fail to prevent the Japanese attack?

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Warning system — warning system U.S. Air Force Boeing E-3 Sentry, an airborne warning and control system (AWACS) aircraft.

The SCR-270 radar system at Pearl Harbor successfully detected the incoming Japanese warplanes on the morning of December 7, 1941, but the observation failed to prevent the attack due to a critical lack of appreciation for the data's significance.1

When U.S. Army Pvt. George Elliott observed a large flight of planes on his radar screen, he reported the sighting to his lieutenant. However, the lieutenant instructed him to disregard the observation, as a flight of B-17 bombers arriving from the U.S. mainland was expected at that time.2

This failure was compounded by broader systemic issues:
* Inadequate Training: Radar training for personnel was in a far-from-advanced stage.2
* False Estimates: Military leadership held a false estimate of enemy capabilities and intentions, believing it was unlikely the Japanese would mount an air attack on the Hawaiian base.3
* Operational Limitations: The radar sets were only being operated on a limited schedule (4:00 am to 7:00 am), and the significance of the radar observations was not realized until the attack was already underway.21