Well, with the impending release of his second feature as Gakuryu it becomes abundantly clear why. He simply wants to make a very different kind of film.
The Flower Of Shanidar plays with genre elements in a manner similar to Aoyama Shinji's Eli Eli Lama Sabachthani in that while they're very much present the genre pieces are handled in a very realistic, arthouse manner. The hyperkinetic style of Ishii's past work is very clearly being left in the past.
Shanidar revolves around a scienctific research team exploring the potential scientific benefits of the titular flower - a rare bloom which only grows on the bodies of a small handful of women. Because the benefits of the flower are so prized the women themselves become very valuable until some begin to show signs of physical abnormalities resulting from the flowers, abnormalities leading to death.
The first trailer has been released and is very strong. Take a look below.