Because the government compels users to use things in transactions. Actually, I should have chosen Chuck E. Cheese's in (2), as there is indeed in that case as

Because the government compels users to use things in transactions. Actually, I should have chosen Chuck E. Cheese's in (2), as there is indeed in that case as well a more powerful entity that forces people to use that currency for the purpose (albeit tokens for games and not money for taxes). Bitcoin has no power that promotes its use in a backed sense, so it's not that kind of currency.