[Haruspex_Pariah]

I think it's just not feasible to keep every Cadian Regiment on Cadia, all the time. Black Crusades are supposed to be rare events, no? Based on how badass Cadian soldiers are, they are probably sorely needed elsewhere, probably against Orks. Of course when a Black Crusade comes down they'll call as many dudes as possible but they can't be waiting on Cadia 100 years a century.