Cheating First Sage.

The first Sage needs someone with whom he can work hand in hand to improve the plane. Preferably someone that knows his will without him speaking about it. Someone that cares deeply about the plane and doesn't need to be baited or bribed to do something for the good of the plane. Someone that can be completely trusted. Soverick is not any of those things. 

Soverick became the child of the plane because he had a use, and he can be removed when they consider him no longer useful. It is not because he is a bad child of the plane. No, it is the fact that there could be a better alternative. Soverick is on the path to becoming a world god, while Salvini doesn't have a definite path to that level of power yet. But Salvini can make that path possible. That's the difference between the two of them.