That's because SMS is a horribly broken, hacky standard, and RCS has to inherit and deal with all the horrifying edge-cases of SMS, MMS, and legacy cruft going back prior to the turn of the millenium.
Then it has to accomodate every other intersted party, many of which hate each other. Apple has always been a bit of an odd duck ("Think Different" has been internalized for some time), but Verizon actively hates OTT messaging as they can't charge for it. Samsung would rather run their own RCS implementation to create and advertise "Samsung RCS", and Google can't push too hard without getting EU attention for antitrust (again).
RCS has been stuck in limbo-hell for years for multiple reasons, none of which are easy.
The specific issue I'm talking about is how Apple for some reason ties the presence of RCS persistently to a contact that requires the user to manually go in and adjust, otherwise the conversation switches back and forth between SMS and RCS as each participant texts back and forth.
This is a problem no other vendors have, and is solely caused by Apple.
It seems like every other message gets downgraded to SMS.