Why was the USA so unprepared for the attack on Pearl Harbor in 1941?

Extract from an interview with a Japanese officer at Pearl Harbor

Catalogue ref: AIR 20/6053

Q. Were you a pilot or an observer during the PEARL HARBOR attack?
A. I was an observer.

Q. How many aircraft used in the attack?
A. A total of 350.

Q. What types were employed?
A. In the first wave:

   50 High Level KATES [level bombers]
   40 UTB KATES [torpedo bombers]
   50 Dive Bombers VALS
   50 VF ZEKES [fighter bombers]

In the second wave:

   50 High Level KATES [level bombers]
   80 Dive Bombers VALS
   40 VF ZEKES [fighter bombers]

Q. How many aircraft were lost. (Failed to return to their carriers)?
A. 29 in all. 9 VF in the first wave and 15 dive bombers and 5 VTB in the second wave.

Q. Which units of the fleet participated in the PEARL HARBOR attack?

   BBs [battleships] KIYE, KIRISHIMA
   CVs [aircraft carriers] AKAGI, SORYU, HIRYU, SHOKAKU, ZUIKAKU
   CAs [heavy cruisers] TONE, CHIKUMA
   CL [light cruiser] NAGARA
   DDs [destroyers] 20 (large type)

Q. How many Air Craft were employed as CAP [air cover in case the Japanese fleet was attacked] over the Pearl Harbor attack force?
A. 50 VP [fighter planes] from CVs [aircraft carriers] plus 12 float planes [aircraft which had floats - they could take off and land on the sea] from the BBs, CAs and CL.

Q. Were they in addition to the 350 planes used in the actual attack at PEARL HARBOR?
A. Yes.