<p>That's the question. Give only one reason you think why have OODB failed or why many systems nowadays still use relational databases.</p>

The main reason is SQL. It is very useful to be able to use the data from a database in other contexts outside of the application, and often with object databases the data is stored in a format that can't easily be queried.  With a relational database the data can become part of a data warehouse, for instance, or just queried by sys admins etc.