Magic? You are talking about a creature that exists as a half-daemon as the result of a human reproducing with a daemon. Maybe these Cambions can't harness the same level of energy required to produce a daemon seed. Or perhaps the Cambions are not born of Human Mothers. Maybe the only way a Cambion can be born is by Daemon mothers.
I suppose this does not answer the specific question of why can't Cambions reproduce at all? I would be interested in the premise where they can reproduce but their offspring would be human and devoid of the daemon magics. Though this could alter the entire premise of the setting. Maybe the satanic society intentionally sterilizes all of the Cambion babies and it is a huge secret to prevent the Cambions from realizing offspring (to maintain control, or something?).
It could be as simple as the half daemon part of them mutates their biology to not resemble a humans. Maybe their reproductive organs don't work or they don't have them 'per say'se' in the way normal humans have them.
Just spit-balling here. If your world is more magical I say lean into the magic. If your world has a strong science aspect where daemon magic can be studied and understood by science then I would say lean into that.