It’s because this season very few new exotic cosmetics. Most are from year 1 and if you already got them your chances of getting a duplicate are decreased.
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I had a check and there is still plenty in the year one engram I don’t have. I think the lower drop rate if true is so that players don’t quickly run out of exotics to unlock as they are not adding many new ones. As you say especially lame for players who already have most of the year one exotics.