Maybe I'm reading too much into this, but it seems like you expect the answer "nothing much" to the first part of your question. I have done nothing more than read lots of articles on KDB's lineage and play around a bit with J, so take my answer with a considerable lump of salt, but my impression is that the answer to the second part of your question is "because there's something 'so special' about KDB"; my understanding is that it provides blazing-fast access to memory-compact databases, from a tiny codebase, building on the APL/J legacy. Why can't this be done in an open sourced way? Well, surely there's no inherent reason, but the fact that it hasn't been is probably evidence that it's not just a problem of trivially cloning existing work (or else someone would have done it).