It might also partly be due to Japan's approach to confessions:

Confessions are often obtained after long periods of questioning by police. This can, at times, take weeks or months during which time the suspect is in detention and can be prevented from contacting a lawyer or family. Thus, since the suspect is put through prolonged strain, stress and pressure, the reliability of such confessions can be questioned. To Japanese citizens and police, however, the arrest itself already creates the presumption of guilt which needs only to be verified via a confession.