Because he's scared, is the short version. Ignatius doesn't show his uncontrolled emotions very boldly, but it's what his reluctance points to. He knows what he could try doing, but there's the chance that he's somehow falling for the Mother's trap. What if fighting her for control of Rita will somehow give her access to him again?

Herbert is lucky that Ignatius owes him for past services, or he would probably be lashing out in his direction right now. What on earth does this man know of these things? Probably nothing.

All he does is give Herbert a cutting look and fall silent.