The directory was unsuccessful and was eventually replaced because

None of the constitution had been created that outlined its authority, is the right answer.

Explanation:

The Directory was a committee of five-member that ruled France in the period between 2nd November 1795 (while it succeeded the Committee of Public Safety) until 9 November 1799, (at the time when Napoleon Bonaparte overthrew it). Napoleon replaced this Directory with the French Consulate. The main reason behind replacing this committee was that none of the constitutions had been drafted that describe its authority.