Carriers supported it. Just Apple didn't, for a long time. The reason is obvious: To increase their market dominance in the US, where iMessage is common. Especially among US teens this was apparently successful. Teens didn't want to be the lame green bubble kids with reduced messaging features. They flocked to iPhones.

RCS is (was) the prime example of Apple's anticompetitive behavior, after the App Store exclusivity, preventing side loading, and disallowing alternative browsers.