@Kathir: There are a few reasons as to why River Safari (RS) is sited between the Zoo and Night Safari. RS was the amalgamation of 2 divergent ideas - an aquarium display and a South American exhibit for the Zoo. The site identified for the latter was a section of the Zoo. As the concept of RS came about with the 2 ideas merged, more land was needed and eventually 12 hectares was set aside, comprising parts of Zoo and Night Safari land.
Second reason is practicality. By putting RS between the Zoo and Night Safari, it could share many facilities with the other 2 parks (administration office, parking lots, utility services and infrastructure, animal hospital, central kitchen, quarantine area, etc).
Third reason is that the authorities wanted to preserve the remaining forests around the existing parks, and so sited RS on land that has already been developed. The forests, though not very expansive, harbour some amazing native species such as colugo, pangolin, king cobra and even sambar deer.