The number of bitcoin is too small to be widely circulated in society. And it exists not as a guarantee of national credit, but as a basis for trust among bitcoin players, so the credit of this "currency" is relatively low.
Moreover, the small number of bitcoins, and their concentration in the hands of a few large players, makes their price highly volatile. That's basically not money. Because an important prerequisite for a common currency is price stability, otherwise the economic order will collapse.