From what I recall from the original white paper, Bitcoin was a name for something. A name is a proper noun. As far as I know, and from what I was consistently taught K-16, that's technically how the official English language works. But then again, the more I think about it, even though another example, AUD (The Australian Dollar) was originally a proper name and capitalized, it does seem that many leave it uncapitalized. I would think it is technically supposed to be capitalized if it can be considered a proper noun. But I suppose many do not adhere to tradition / rules of original proper grammar.