Probably just because the devs didn't like the look of the originals and it likely didn't take very long.
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No matter how long it took, it was a waste of time, and needless. If it was 5 minutes, that 5 could've been spent making Bad JuJu's skull glow less intense, or any of the numerous issues with many aspects of game. Pointless change is pointless.
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No guarantees on how long it took, people have posted some relatively simple things on the forums and bungie has replied saying that they might take a while to implement.
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Swapping images and changing coding are two completely different projects.