No good reason, except that I wasn’t really aware of these two 🙂
I looked for an object persistence layer, and an Apache project seemed like a good idea at the time.

I’m using it now and after you get it configured, its pretty useful.
To really answer your question I’d have to work some time with both of the two products you mention and compare them, which I don’t have that much time for, right now.

From reading a bit about JDO I get the idea that its just an API – like JDBC – without any actual implementation ?