This is just my theory, but imo it's about the habit of the consumers, which is being dictated by the carriers.
Eg. in the US, most wireless plans that I see offer unlimited voice and text, and whoever you call/text, it's the same rate/included. Thus the market itself doesn't have the habit of looking for a more economical route of calling/texting. The genius of iMessage is that Apple baked it in to the stock SMS app, so there's literally no transition from the iPhone users. They just start using it right away.
This is different in other countries. I'm taking my country Indonesia as an example. Here, the carriers don't offer unlimited calls/texts. Even worse, calling/texting to different providers or landline will incure different (more expensive) rates than calling/texting to people within the same carrier. Thus the market behaves in a way that we naturally are looking for more economical route. Prior to smartphones/data plans, the options are either calling or texting, thus we adopted texting way ahead compared to the US as texting a single SMS is cheaper than calling the person for a few minutes. In the era of data plans, whatsapp (along with the rest of messaging platforms like Line, Telegram, Wechat, etc) immediately became the choice as calls and texts are "free" as it's using the data plan. So people has no problem looking and downloading those apps on first time set up as they already develop the habit of using a different app than the default call/SMS apps. Even iPhone owners here don't bother with iMessage as majority are on Android.