Why Do The Japanese Sleep On The Floor?

The Japanese have traditionally slept on a futon on the floor. Before you go to sleep, you take the futon out of the oshiire (closet) and spread it out. When you wake up you have to fold it together and put it back into the closet. That became such a routine of mine that I now do it almost automatically. But times are changing and less and less people sleep on a futon. When I asked my Japanese classmates only 25% said they‘d sleep on a futon. And also only two of the six host families I stayed at had futons.