Better security. Google's version of RCS has end-to-end encryption, which Apple does not intend to use. Apple will instead work with the GSMA to develop a more secure form of encryption that is baked natively into RCS.

Basically Apple is going to make RCS better than Google ever hoped for, in other words, RCS on the mobile carriers side will become E2E by default, and doesn’t have to go through Google servers.