That's a good question.
Although we haven't ruled out the possibility of buying a duplex, they have two special characteristics. They're often more expensive, because the owner has a private address and is also responsible for maintaining the outside, not just the interior. So a duplex is often very expensive.
Furthermore, since they're often located at ground level, the many security-related renovations we would have to complete would be quite costly. That's why buying a duplex is not very attractive.