It's completely random. My first Titan before I sold my game back in December had a Dead Orbit Rank in the 40's and I never saw a single Dead Orbit shader, let alone the highly coveted Revenant shader. My new Titan which I started back when I repurchased the game right after House of Wolves dropped just reached rank 15 and I got Bittersteel at 14. I'm amazed you don't have it yet.
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I'm also really amazed I thought that teaching rank 81 in any faction would get you all their loot.