I have been using db4o (an object oriented database) lately on my personal pet projects, just because it is so quick to set up & get going. No need with the itty, gritty details.
That aside, as I see it, the main reasons why they haven't become popular are:
- Reporting is more difficult on object oriented databases. Related to this, it is also easier to manually look into the actual data in a relational database.
- Microsoft and Oracle base so much of their business on relational databases.
- Lots of businesses already have relational databases in place.
- The IT departments often have relational database expertise.
And, as Jan Aagaard, have pointed out, lately it is because the problem have been solved in a different way, giving programmers the object oriented feel even though they program against a relational database.
