So, you definitely have a better imagination than I for possible places to use this development model. That said, the areas you've proposed, as others have mentioned, are bigger problems to solve, sometimes much bigger. They require a lot more capital-intensive pieces to come together, or they require accurate models that don't necessarily exist.
Of course, that didn't stop companies from competing for the X-Prize. So, perhaps in the areas you suggest the prize just needs to be bigger (maybe a lot bigger). So, while an improved query optimiser for Oracle might require hundreds of thousands of dollars worth of hardware to develop, a $10 million prize would still be sufficient to interest some teams. At some point, the line is crossed between what is a good development+marketing investment, and what the company could do with its own in-house developers. I don't know if $10 million is past that line, or if it's enough incentive to bring out the geniuses in the field (the ones that don't already work for Oracle).
Oracle would also have the problem that they'd have to create some sort of stripped version of Oracle that only works on this one data set, or something, because otherwise they'd be giving away the biggest license of their software to any developer that signed up (because to test major improvements to Oracle, in the places that matter to Oracle, you need to be running it on big MP machinery with terabytes of data).
So, maybe that's why more companies haven't done it, despite the apparent cost-effectiveness and marketing value (NetFlix has probably gotten a million bucks worth of tech press out of this contest). The cost of getting people on board might be much higher, and the cost of providing the tools to contestants may also be dramatically higher. There may also be negative media consequences. NetFlix spun it beautifully...such that we think of them as very smart scrappy guys and hard to beat, but also eager for a challenge from upstarts. Maybe a big behemoth like Oracle would come across as, "Oh, now they can't even be bothered to build their own software, and expect a bunch of Open Source experts (since Open Source is where a lot of the expertise in databases currently exists, outside of companies that wouldn't want to help Oracle) to do it for a pittance." So, this kind of thing might turn out to be negative publicity for a big company, where it is great publicity for a small company like NetFlix.
Just some devil's advocate stuff. I think you're probably right that there are other areas where this would work. I don't know that you or I have thought of the best examples of them, though. Maybe someone reading this thread might have a company for which this model fits perfectly, though.