What is a rest room for Japanese? It is a private space for relaxation. A rest room is completely separated from other rooms. Because of it, Japanese who are easy to stress in human relationship can relax in the room because they can be alone for a while.
Nowadays, many toilets in Japan are equipped with Japanese-style-bidets, called in Japan “washlet”, and have a function to warm a toilet seat, etc. In fact, 20 million washlets was sold as of June 2005. See the first graph in this cite.(I’m sorry for not finding English cite, but I think you can see what the first graph displays.)
I reckon one of the reasons that Japanese toilet developed is Japanese orientation toward purity. However, it would be also connected that Japanese try to make the room more comfortable. A rest room for Japanese is not just a rest room.