It's not such a mystery. WhatsApp came out in 2009. RCS would not be rolled out widely on Android until 2019. RCS was not implemented properly on iOS until literally a month or so ago.

Therefore its competition has been SMS mainly, which it is directly better than in every way. SMS is not encrypted. WhatsApp is. SMS (technically MMS) heavily compresses pictures & video. SMS splits up long blocks of text into multiple texts, which may or may not come in the correct order. SMS used to be limited in cell phone plans around the time of WhatsApp's release, and STILL is in some parts of the world. SMS requires a consistent cell signal which requires a paid monthly subscription, WhatsApp only requires internet. Sometimes it is easier to find public Wi-Fi than a good cell signal in some places, and it was a popular thing for some teenagers to get an iPod touch or parent's old phone and only be able to use it with Wi-Fi. Then there are stickers & video chat.

People got used to it being the best for at least a decade, and don't want to switch. Even nowadays, RCS is nearly as closed a standard as WhatsApp being limited to an app. You have to use Google Messages or iMessage specifically. In Apple's case, your messages aren't encrypted on their servers unless you specifically request it, and they hold the keys. I suspect Google is the same, despite what they say. And RCS still is routed through your carrier and the receiver's carrier, who have varying policies.

Of course, WhatsApp doesn't have zero access encryption for their servers either, and they're ran by Meta, so it's nearly a lateral move at this point. The real solution is a better WhatsApp replacement like Signal. Signal is making progress but it takes time.