Because the government very rarely allows ruminant ungulates allowed into the US nowadays. They're overly concerned about the spread of disease into the food supply. This is one of the biggest problems facing the future of even-toed ungulates in US zoos.
~Thylo
Interesting. That seems like a very avoidable problem (as in making sure that the exotic hoofstock never come in contact with domestics) and a typical government overreaction.