Even though an automotive manufacturer may have dozens of models of vehicles, many of them share the main engine design. In particularly compact and low cost vehicles the engine is often chosen because they already make so many of them that it's the cheapest engine that meets the requirements.
This is particularly true when a new model is being introduced.
Without looking further into the specific details of these vehicles, it is quite possible that the engine chosen for these models as well as the engine configuration in conjunction with the transmission and drive components, necessitated a particular exhaust manifold design. Otherwise they would have had to redesign significant parts of the engine. This would raise the cost of the vehicle, reduce its reliability, and increase the service cost due to additional new parts that service places would have to store, order, manage, and install.