Or perhaps just a mistake made by bad management
It was undoubtedly a mistake with the advantage of hindsight given that all three of those adult bulls have since passed away and the exported hippos have since been identified as being from a valuable line.
Looking to the future, I hope once imports open up, zoos take full advantage to expand the genetic diversity of the regional population. All of the hippos in the region descend from Taronga stock in some way.