I think you have largely answered the puzzle with your comments above. The great majority of Shikoku pilgrims travel by bus or car. The number of Japanese walking pilgrims is very small. In recent years the Shikoku pilgrimage has become well known amongst foreigners like myself and those foreign pilgrims who follow the route do mostly choose to walk. Many of us having already walked one or more Caminos. As I understand it the shortage of accommodation is largely due to the combination of increasing numbers of foreign pilgrims and a reduction in available beds post-Covid.