Willie the Duck
Hero
I am guessing it is to avoid having massively multiple anthro-style PC races <edit: of the same IRL animal>. Obviously D&D has never shied away from going to the same well multiple times, but I imagine they don't want the designers of each setting having to try to figure out a place to put Grung, Bullywug, and Grippli into their campaign setting (in PC-adjacent ways).
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