Like most things Japanese, porno in Japan has its own unique history and sub-genres. Here is a great article on the history of Japanese AV by Johnnes Shonherr in The Midnight Eye:

"Onanie became a surprise hit and attracted the attention of a new group of investors who had so far been rarely attracted to film. The publishers of ura-bon (illegal underground sex books) and bini-bon (vinyl-wrapped porn books) thought that they had just discovered a gold mine and went right into the business, forming their own AV production companies."