There is one very important thing you are not considering. The Central Bank has to oblige itself to accept its own liabilities to cancel debts with the Central Bank, otherwise its liabilities would be worthless. The currency issued by the Central Bank is a liability because it is an obligation.
Bitcoin, on the opposite, does not represent the obligation of anyone. No one obliges itself or is obliged to accept Bitcoin. Bitcoin is a property ledger, not an accounting ledger.