Around the time smartphone and widespread data coverage made it feasible. It, and other services like it, offered a clearly superior experience to SMS/MMS, and RCS arrived way, way too late. As for why a particular service succeeded in a particular market, I imagine it all comes down to marketing, luck, and network effects. WhatsApp is probably not my ideal service (mostly because it uses somewhat device-tied accounts) but it's what 90% of people use

Almost all phone calls I receive are "actual" phone calls though. And SMS still gets plenty of use as the lowest common denominator for new recipients, but in practice over 95% of my received SMS are from computers now