I got the impression Volvo was able to do this for fairly small production runs, whereas the Japanese figured out how to consistently do it in large volumes.
That may well be so, Japanese cars had an absolutely terrible reputation for being rustbuckets in the 70's so they had to do something about it. Given the number of them that were manufactured the fact that they are so rare today is as far as I know uniquely due to the rust problem.
I'd say the Japanese figured out how to make this happen on a cheap car. Almost all high end european cars (including Volvos) had great rustproofing for a long time but cost far more than Japanese cars. The Japanese made the first reliable and long lasting low priced cars.
