WhatsApp works with your phone number. If you have someone's number, you have their WhatsApp. And since basic text messaging is terrible and RCS still isn't universal, WhatsApp is used.
Because WhatsApp is really good, much better than SMS, and everybody uses it.
Meta only bought it after it was already the de facto standard. And to be fair they are only just starting to ruin it after quite a few years. So I would say the world made a pretty good decision there.
Sell a way for businesses to send trusted communications to their customers in sensitive industries - i.e. healthcare would be a big one.
They need both an actual revenue stream, but also that sort of professional messaging can drive adoption which ultimately furthers the Signal mission.
Plus all those things could desperately use good secure messaging systems.