Jonathan Robie, in an interview with Jon Udell, captures the reason for failure of the object database market better than any explanation I've seen...
JR: OQL is a nice language. The grammar of XQuery has benefitted a lot from it, and some of the implementation has too. Of course, it wasn't commercially relevant for the object database vendors to implement OQL in any kind of complete way.
JR: Every object database had a different market, and a different model, and people weren't looking for interoperability between ObjectStore and POET, because why would you use them for the same thing?