My guess is that it’s because ES6 promises weren’t available back in the day so RSVP was added as the promise library of choice. I’d also assume that the core team intends to move away from RSVP promises in favor of ES6 promises (it seems they prefer to stick close to js core when possible as opposed to including extra stuff) however that can take time. There’s a lot of stuff to consider from a framework standpoint in terms of backwards compatibility, upgrade path, and of course all the work to actually implement the switchover.
So in summary: this is just a guess on my part but I’d assume that the intent is to migrate eventually, but that takes time. I didn’t look to see if any RFCs or projects were already underway, but that might be the case also.