I actually thought about this thread while in bed last night. I think one of the key reasons (in addition to what Kifaru said above) why there is not much emphasis on European species in American zoos is ---- perception. When Americans think of Africa, Australia, South America, and much of Asia, we think of the wild places -- the African savannah, jungles of Asia and the Amazon, and the Australian outback. But when we think of Europe, we think of cities and people, not animals. If you asked the average American to name 3 animals that live in Europe, most would probably say "Dog, cat, and horse".
Another example of perception here in America is the lack of Central American animals in our North American exhibit areas. There are excellent North American areas in North Carolina, Columbus, Minnesota, Fort Worth, Oklahoma City, Oregon, Lowry Park, Tulsa, Sedgwick County, and elsewhere. But none of these North American areas feature spider monkeys, tapirs, toucans, or any other animals from Central America. Why? Because the "perception" is that Central America is more like South America. In fact, one recent poster even argued with me that Central America is NOT even a part of North America -- which of course, it officially is.