This is the classic "innovators dilemma". My 2-cents: Kx has a good business selling kdb+ licenses to the big banks and trading firms. They've been exploring how to expand beyond this niche, but many big data/analytics startups go straight for the FOSS solutions. In addition to the cost/licensing advantages of FOSS, a large user-base and ecosystem has developed around things like mongoDB, giving them compelling business advantages over kdb+ (even with kdb+ performance advantages).

Although mongoDB is a demonstration of how to build a successful business around open-source, it's very different to start out that way rather than move that way after you're already established as closed/licensed product. Kx would have to essentially kill-off their existing profit stream with the goal that they could build a larger business around FOSS kdb+. That's a risky proposition, fraught with many pitfalls.

It is a shame they backed-off of the fully-open 32-bit version. At least that had some potential to spur some user-base/ecosystem growth (at a minimum, this could have encouraged the development of novel clients/editors/REPLs/debuggers/charting/etc...), without threatening their core profit stream.