I can think of several possible factors. One is that, in the 1950s and 1960s, bikes were seen primarily as practical transportation, not as exercise or sport. Many of us started as paper boys riding with shoulder bags so heavy we could scarcely walk. We all rode to school, as a matter of course. We rode everywhere until we turned 16, then started again when attending college, because students were not allowed to have cars. In graduate or professional school, we had cars, but there was no student parking within a half mile of the department. The result was that cycling was natural to us -- not a statement, not a green thing, not exercise. In some ways, we were like today's Dutch.