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.