From my understanding, the reason RCS was delayed is because Google's RCS was E2EE only in certain cases (both users using RCS). But also because Google's RCS runs through Google servers.

If Apple enabled RCS in messages back then, but the recipient was not using RCS, then Google now has the decrypted text message, even when RCS advertises itself as E2EE. With iMessage, at least I know all of my messages are E2EE when I see a blue bubble.

Even now, RCS is available on Android if using Google Messages. Yes, it's pre-installed on all phones, but OEMs aren't required to use it as the default. It opens up more privacy concerns because now I don't know if my messages are secure. At least with the green bubbles, I can assume that anything I send is not encrypted. With RCS, I can't be certain unless I verify the messaging app the recipient is using and hope they don't replace it with something else that doesn't support RCS.