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Bitcoin is Stupid and Does Not Deserve an Emoji (blog post)
(thedabbler.patatas.ca)
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I saw this get brought up a lot. I think the difference is that currency symbols generally don't refer to a specific currency. USD and AUS both use the $ symbol, for example. "Dollar" and "American Dollar" aren't the same thing since other types of dollars exist, and the symbols are still technically multi-purpose, whereas the ₿ symbol technically refers only to Bitcoin.
That's my theory on the reasoning, at least.