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My company is a homebrew 21st Founding chapter that, immediately upon leaving for their first campaign, got lost in the warp, were savaged by Daemons, and lost their Codex Astartes before reappearing on the far fringes of Imperial space. Those who survived were badly wounded, but with an extremely large supply of tech and supplies lying around to rebuild. (Represented by Iron Hands.) Their new recruits, though, are more wild and prone to aggressive mutation. (Represented by Space Wolves.)
I therefore think that getting lost in the Warp is an excellent excuse for having old tech.
Also, bear in mind: Mkiii armor, at least, is worse than other Power Armor in the fluff. The frontal armor plates are thicker, but its slow and the backside is vulnerable. They didn't get phased out due to scarcity, they got mothballed for a more reliable design.
Fluff wise, it's mostly used for fighting in corridors and caves and other enclosed areas, where they don't have to worry about being flanked. If your chapter (or part of it) just specialize in narrow, close fighting, it would be extremely sensible for them to have Mkiii Armor.
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