I have no idea. But there's something in the English language that brings out the muthaf#$#a in all of us, as Steven Pinker reminds us, by way of Andrew Sullivan. As opposed to Japanese.
Do you have any idea?
ADD: Two observations:
Steven Pinker doesn't take note of the fact that almost all swearing consists of Anglo-Saxon words.(He is a linguist, not a philologist.) In fact, in the examples he gives, Jesus Christ is the only one that uses words of Romance language origins. And Anglo-Saxon words are short, as are basic vocabularies; thus, the four-letter word.
In Japanese, the three most commonly used phrases used in situations where an English-language speaker would use swear words and phrases are variations of: kono yarō (this low-life man), chikushō (non-human animal life-form), and kuso (dung). Basically, they are no more than vulgar epithets, and are heard commonly on TV broadcast programs. They do not seem to carry the sense of taboo that continues to lurk behind the pungent parlance of that most permissive of societies, English-language America.