Neither Indus not Atlas develop the car themselves. They purchase ECUs from the international sellers, ie, Honda and Toyota. Since the international version of Yaris has VSC, local Yaris has VSC. Since City Thailand does not have VSC, local City does not have VSC.
Iss mein Honda aur Toyota ka koyi kamaal nahin hai. They just make use of what's available in the international market.
With that said, I completely agree with you regarding the importance of VSC. Mind you, most people still have cars without VSC, and VSC has a minor impact at best at low speeds.