I rushed to just grab that codeblock from Wikipedia. But the selection of which characters are considered emoji is not arbitrary. The Unicode Consortium (their Unicode Emoji Standard and Research Working Group to be exact) publishes those list and guidelines on how they should be rendered. I believe the most recent version of the standard is Emoji 15.1.

Edit: I realized I’m going off track here by just reacting to comments and forgetting my initial point. The difference I was initially alluding to is in selection criteria. The emoji. for assigning a character a Unicode codepoint is very different from the criteria for creating a new emoji. Bitcoin has a unique symbol and there is a real need to use that symbol in written material. Having a unicode character for it solves that problem, and indeed one was added. The Emoji working group has other selection criteria (which is why you have emoji for eggplant and flying money, and other things that are not otherwise characters. So the fact that a certain character exists, despite its very limited use, has no bearing on whether something else should have an emoji to represent it.