Honour the wargear of the dead. "The spirit of this Astartes and the spirit of his armor has been left unatended long enough. Come Brothers, let us take our forefather home so that they both may rest in the halls of the honoured dead."
First power armor "Thunder armor" was made 23k and "Iron armor" was made during the Great Crusade which began around 30k. Oh and when you say Iron men, it refers to servitor robots with über-high-tech-AI, during the Dark Age of technology, which then rebelled agains mankind.
I don't think they rebelled. Looking at a Gaunt's Ghost novel with an STC spewing out chaos corrupted Men of Iron, I think that is what happened during the Iron War. Think about it, the Men of Iron were defeated by psykers. Psykers were then given free reign to develop their powers unhindered. Those psykers then became warp portals for daemons during the latter part of the age of strife to ravage countless human worlds. Then chaos tainted men of iron are discovered popping out of an STC.
Men of Iron thought themselves to be superior since they did everything for humans hence rebellion against inferior beings. And that STC had been in a Chaos-tainted plant for 1000 years. It too could be that Chaos gave a little subtle nudge to Men of Iron.