In many cases the US were the first to mass-manufacture things to some kind of standard, so the question is really: why did the rest of the world choose a different standard. Often they have a good reason (better design, maybe), but the US also has a good reason for not switching (inertia, existing tooling, people understand the existing standard).
The US was the first place that traffic light and crossing signals were installed. The reason they didn't go with the standard the rest of the world uses is that it didn't exist.