History Of The 25-Year Rule
Before 1966, there were no federal safety standards for vehicles in America. States set individual laws, which were pretty similar across the board, but it was essentially a free-for-all where you could get away with just about anything.
Post World War II, there was an influx of foreign market cars and bikes on US shores as returning soldiers brought back Italian sports cars, British motorcycles, and the like that they had experienced while on active duty (Japanese cars were shunned, naturally, and only became popularized later on through much hard work from the likes of Honda). But as more cars populated American roads, road deaths increased, prompting safety concerns from the government.