not for no reason, no. They aren't allowed to be imported because there isn't an import standard for them (which is a legal document detailing, among other things, the prevention of diseases or other risks to the country's native wildlife and economy from that species). If zoos want to import hippos then they need to apply to have the legislation put in place to allow hippos to be imported. No animal can be imported unless there is an import standard for that species. If it can be shown that adding a species to the allowed list has no biosecurity risk, then it may be added (or it may not).