When you consider that a lot of Japan's traffic is slow moving arteries on account of massive population density, collisions are likely to occur often but not as deadly. I suppose that causes people not to take seatbelts as seriously.
In my home state, it was not required by law for people in the BACK seat to wear seatbelts. Redneck Roadrage and Redneck drinking made this law (lack thereof) a REALLY bad idea. So it was only changed in the past decade.
