Just because OODB are not the mainstream we should still consider the successes that they have had. Cache and Zope are both widely used (relatively) but would be considered successful by some standards.
Perhaps the biggest reason that OODB have not taken hold dramatically is because of the success of the hybrid object-relational systems that take most of the potential marketshare from OODB: PostgreSQL and Informix.
I know that this does not directly answer the question but it is, I think, part of the equation. Overall, though, I think that momentum and the heavily ingrained thought processes supporting relation databases make it difficult for people to switch. Currently the DB profession is trained almost exclusively in relational theory making your DB professionals very interested in avoiding OODB and academia teaches DB theory for practitioners almost exclusively on relational.
Until large, corporate DBAs and mainstream professors and curriculum and turning out staff beyond developers prepared to managed OODB I feel that it is unlikely to see mass appeal no matter how good it is from the development side.
