I haven't looked at every comment to see if anyone has stated the obvious: search is a self-fulfilling prophecy. The more data you have, the better your results. The better your results, the more data you get.
No one will make up ground on Google until a radical new algorithm is invented, and Google doesn't buy it :)
I don't think it's a matter of data access. I would argue that Bing has parity with Google in most respects (they have the same Internet to crawl, partnerships with some data providers for things like flight search, etc.), and they even have decisive advantages in other areas, like access to huge piles of social data. So why aren't they as good or better?
No one will make up ground on Google until a radical new algorithm is invented, and Google doesn't buy it :)