Very simple. The Japanese police is one of the most efficient of its type in the world. They have around 99.8% conviction rate, for all crimes.

http://www.jstor.org/discover/10.1086/468111?uid=3738256&uid=2&uid=4&sid=21104123198157

Also there are several other facts, such as lack of immigration, ethnic homogeneity of the society.etc