Not sure, but it can be suggested that water tanks on rooftop began to show up when reinforced concrete buildings populated in the island of Taiwan, around the 1970s.
Possibly driven by the wants for secured and clean water, rooftop water tanks have been a major part of Formosa landscape (all islands included), since numerous natural hazards, e.g., typhoons, earthquakes, or droughts, and lack of environment managements easily affect the quality and stability of tap water in these islands of tiny size.
Also possibly because flat roof buildings' potentiality for increasing storeys and sanitation, portable tanks are much popular than installed ones. Regardless the history that the 921 earthquake in 1999 overturned all portable tanks within the reach of its shock wave, the legend lives on.
All kind of materials has been tried. Glass/Fiber Reinforced Plastics were used. Now stainless steel holds the majority. What ever the material and shape may be, they all look just like something comes out the childish world of sci-fi.
Adults are much fascinated by something like Hollywood harrows that human body being brewed in water tank for months...