What was the reasons given by Japan for not invading the U.S. during World War 2?

Actually, the Japanese did invade the U.S. :D - check out the Japanese invasion of Alaska and the Aleutian Islands Campaign.

The reasons given for not invading mainland USA were supply problems. Japan had no solid base for a massive invasion across the Pacific like that (remember, US forces had to island hop just so they could invade Japan) and were at the time fighting many different battles in Asia. It was also never part of the Japanese plan to invade USA, rather create an iron wall of defense surrounding Japan, the Dutch East Indies, Phillipines, etc. and to let the allies crash up against this wall.