I've often wondered why there aren't more 4/8 doublers and wonder if it might be that the scholastic and collegiate schedule don't allow for much time between the events.
Could also be that the 400m is typically considered a sprint, while 800m a mid-distance event, and in the american system, coaches tend to specialize too.
In some states that allow for high schoolers to double (Montana is one) there are a number of very successful 4/8 runners. Obviously this is different from elites.
There's no reason why there shouldn't be more capable 400/800m doubles. Mark Everett was probably the last true elite US guy, earning WC Bronze and running in prelims of WC 4x4. In the US, where there are tons of world class 400m guys and very few wc 800m guys it would make sense for some of the low 45 and high 44 guys to try to move to 800m in order to make it to the next level.