Uh, I’m really not sure what you mean here:
There’s no 40K: Rogue Trader era thing where criminals plus power armor equals space marine.
The equivalent IE equation would be “criminals plus Iron equals Lord-Pilot Anvil.” What both settings lack is someone who uses the power armor, yet is not of the power-armored caste.
You know how they do the gene-seed magic and slap the armor on you and then you’re not “a criminal with really big shoulderpads,” you’re “a Space Marine?” Same thing here. There’s extra cultural baggage attached to the Iron that makes you a Lord-Pilot Anvil. (Notice that it’s not “CEO-Pilot Anvil” or “Preacher-Pilot Anvil” instead).
Minor quibble: Illegal Crucis does not allow you to use Iron. Iron requires the Iron Trained trait, which in turn requires Corvus and Crucis specifically. Illegal Crucis just gives access to the Helm skill.
Most Magnates probably don’t join the Pilotry at all, and you may be right that of those that do, most join the Hammer. But there are reasons to go Anvil instead. I suspect that it’s not generally out of a desire to be Lords-Pilot Anvil, but rather out of a desire to become Anvil Lords or to become Forged (if the lord you plan to follow on crusade favors the Anvil, he’s likely to hand out more fiefdoms to his Anvil vassals than his Hammer Lords). But if you don’t see it, that’s fine. The prereqs in the LPs show what’s possible, not what’s common or what’s a good life choice (you’ll note that Magnates also qualify for the Filthy Worm Lover and Clown lifepaths).
I think you’re applying your own values to this, though. You come from a place where it’s better and higher-status to be a rich, successful man of business than to be a fighting man, sworn to the service of a lord. Most people in the Iron Empires, though, don’t agree with you. To them, the richest Magnate, owner of a dozen ships, is just a merchant. He has much coin, certainly, just as a farmer has much manure. But a Lord-Pilot, even one without lands or title, that man is a noble! He has a quality no amount of money can equal. So joining the Pilotry is a step up, and if you lack the technical skills and training necessary to helm a warship (as many magnates do), you might join the Anvil instead.
Let’s give an example: if Bill Gates or Warren Buffet walked up to our landless Lord-Pilot Anvil and said hello, they’d be cuffed to the ground for having the gall to address a superior without being spoken to first. (An IE magnate would have engineered an introduction, or approached through friends, in such a way as to satisfy the formalities while still making it clear that his status was lower but his power much higher than the Lord-Pilot’s.)