Because contact with human is polluting, God uses demons as mediators since they are already wicked anyway. And yet, this solves nothing (how can wicked beings mediate?). What we find is that when God reaches out to us, we are made pure while He is left unsullied, just as when the sun shines on the earth no damage is done to the sun's rays.
What we need if we are to be saved is someone with the holiness of God yet the body and soul of a man. We need no other than Jesus Christ, the Incarnate Son of God who took on flesh and yet was without sin.
We need a Mediator who, being united to us here below by the mortality of His body, should at the same time be able to afford us truly divine help in cleansing and liberating us by means of the immortal righteousness of His spirit, whereby He remained heavenly even while here upon earth.There is no intercession for us that does any good other than that of Jesus Christ.
The demons would lead us astray by promising a physical unity with God, when in reality our unity is of a spiritual nature ("for we must rise to God not by bodily ascent, but by incorporeal or spiritual conformity to Him,"). Again, there is no true mediation other than through Jesus, who alone stands as intercessor between God and His people.
There are no good demons, period.