It has taken _years_ to get carriers on board with RCS (and RCS requires their involvement) and it doesn’t support all of iMessage’s features (e.g. only supports end-to-end encryption in one-to-one chats, and that after a near Herculean effort from Google). Moreover, the fact that it’s an open standard means that it will likely take quite some time for the protocol to support new features that OS vendors like Apple might like to add, and that that process will be fraught with compromises. It seems completely reasonable to me for Apple to not want to tie themselves to the whims of carriers (who, let’s recall, were charging _absurd_ amounts for text messages before proprietary systems forced their hand) or Google (look at what’s happened to web standards under their stewardship) when it comes to managing their messaging service.
That said, as an iPhone user, I _would_ love to see RCS support _alongside_ iMessage to improve the experience of communicating with Android users, and that actually might happen someday.