OFA isn't something we're able to just turn on and it works. If we went that route, we'd need to dedicate dev time and resources to making sure the new champ is even playable in that game mode. And the team that would be giving those resources is the same one that is working on the different modes for live. This means even less RGMs or lower quality RGMs than what we put onto live.
This is only the current state of where resources are put and could change in the future when priorities change, but for now OFA just for PBE champ releases isn't within budget.