That's the question. Give only one reason you think why have OODB failed or why many systems nowadays still use relational databases.
Can we answer more than once? Another reason is that relational DB's have a strong foundation in mathematics: from the definition of a relation, right through to the normal forms, the theory is rock solid. It is true that the relational model does not map well to OO, but IMHO the benefits and stability of that model outweigh the mapping problem.