Why, generally does an object-oriented system use the relational DBMS?
The reasons for using relational DBMS for OO systems could be any combination of following:
a. Many organizations have existing relational databases including existing business data.
b. Most commercial RDBMS come with "Object-Relational" extension which implements object database on top of a RDBMS.
c. Purely object databases are too complex to use, and lack standards like SQL.