There's no evidence that japan has lower testosterone than an average country. The only study I could find suggested it had a higher average than the US.
That supports what others have commented, which is that obesity is likely the main culprit, at least in terms of low testosterone. The US has a much worse obesity problem than Japan does.
I don't see why, there's a physiological basis for it. It's well known that fat cells have higher level of aromatase which converts testosterone to estradiol.
