The reason for this is that the US was very lucky. None of its aircraft carriers were in harbor on the date that Pearl Harbor was attacked. This was just luck, not the result of any sort of plan.
At the time of the Pearl Harbor attack, the US had three carriers in the Pacific Fleet. One was USS Enterprise. This carrier was a mere 215 miles from Oahu when the attack happened. It was on its way back from a mission to Wake Island. A second was USS Lexington. This carrier was on its way to Midway Island at the time of the attack. The final carrier was USS Saratoga. This carrier was in San Diego at the time of the attack, getting ready to take on airplanes to bring to Pearl Harbor.
Thus, by luck, no US aircraft carriers were at Pearl Harbor to be sunk.