WBC doesn't like their belt in the hands of Russians, for political reasons likely, but also they don't have as much control of their champions and have to play by their own rules. Obviously, Wilder gets preferred over Povetkin in this regard. As for Stiverne, he's managed by Don King, who is good pals with everyone at the WBC, and didn't want to see his Stiverne eaten by the lion Povetkin. It's all a charade really. There never was an intention to let Povetkin fight for this belt the worth of which is only "leverage" not pride, not prestige, not intrinsic value.
I have said this for a very long time now that fighters and their managers with the promoters need to re-think this business model out better. Too much authority is given to these self-authorized boxing organizations who really bring near nothing to sport. In so many ways these organizations are not needed and do more harm than the tiny bit of good they may toss in once in a while. None know and understand this better than Tyson Fury. Yet, so few, seem to know or do know this.