New levels and maps are largely self-contained, so you can write a strategy guide that solely covers the DLC content.
Fighting game characters don't stand alone- they interact with vanilla characters (usually violently), so it makes sense for all the strategy guides to be lumped together so they can cover all the combinations.
And that's the best reasoning I could come up with.
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"I'm Commander Shepard, and you're standing in the biggest library in Citadel Space. Look me up."