I think the dichotomy is also due to the difference in railroading culture between Western Europe and Japan, and North America and Russia. (Places like China, Australia, Eastern Europe, and India fall somewhere in the middle, in my eyes.)
In North America and Russia, cars have to stand up to intermingling with 150 ton(ne) freight cars, which forces heavy frames and heavy skins. That forces a higher price point, which forces North American railroads to use them for longer, and requires more durable materials to make sure these cars won't fall apart. In that kind of market, an all-stainless car is a good investment, particularly when there may not be ready money for another investment in cars.
In Western Europe and Japan, there is more continual money for railcar investment (or so it seems to my eyes), far less intermingling of freight and passenger equipment, and lighter trains all around. After World War II, the systems of both areas were destroyed and rebuilt from nearly the ground up, which meant both a huge demand for new cars to be cranked out with minimal cost and material use, and few heavy legacy cars as in North America and Russia. The proportion of car usage on a daily basis is far smaller than the US, and the need for public transportation is correspondingly much higher. Finally, the rate of advancement in railroad design meant that it was much easier for a car design to become obsolete. Couple that with the generally-milder climates of those two areas, and lightweight aluminum and carbon steel cars make a lot of sense. Stainless would have been just too expensive.