I did the backstage encounter with the Orangutans last year and was told that at the time, Kiarni was on birth control tablets, as they had no intentions of breeding her and Santan. They found out after a while she was just putting them in her mouth and not swallowing them, then giving them to Santan, and that is how Gabby came along.
That would make sense - Maimunah was imported in 1992, so it’s possible Melbourne were just awaiting her arrival as Santan had only arrived five years prior.
Melbourne also only had a single Orangutan habitat at the time - so would’ve had to keep Bono, Kiani and Santan together, which wouldn’t have helped the prevention of breeding.