Why aren't OODBMS as widespread as RDBMS?
Why are relation databases more common than object-oriented databases?
If the Object Oriented Programming paradigm is so widespread, shouldn't we see lots of OODBMS? Wouldn't they perform better than the RDBMS+OR/M?
Answer
One reason that RDBMS has retained popularity is that it's established technology, well understood and has a standard language (SQL) that multiple vendors support. It also has a few good interfaces like ODBC and JDBC that make it connect with different languages pretty well. A stable API is a strong factor in keeping a technology dominant.
By contrast, there is no clear model for OODBMS, nor is there a standard language, nor is there a standard API. There's not even a de facto standard by having a leading vendor implementation.
The OODBMS concept might perform better than RDBMS+ORM. It depends entirely on the implementation. But it's also true that OODBMS don't solve the same set of problems that RDBMS are good at solving. Some data management tasks are much easier if you have referential integrity and relational headers enforced by the data management solution. These features are absent in the OODBMS model (at least so far).
There's a lot of noise on blogs that relational databases are obsolete, but RDBMS are nevertheless the best general-purpose solution for a great majority of data management tasks.