it is not that easy to be a global currency because all aspects must be met if bitcoin want to be global currency, I think bitcoin is not yet deserved to be a global currency because many aspects that have not been fulfilled ..
which aspects you are talking about , it serves most of purpose of a currency .
we can store it , transfer it , utilize it as mode of payment , have a value (volatile though) . so it is already a currency , only thing is it is not legal in all countries . apart from it tell us why it can not be global currency ?