That's probably because it's not worth nearly as much to MS or Yahoo. I'm sure a non-trivial percentage of users would change the default search away from Bing.

Also, with Yahoo's search results now coming from Bing, this does mean that the biggest potential sources of funding are all linked to companies who compete directly with Mozilla's biggest product - Google and Microsoft have their own browsers. It's certainly not an ideal position for Mozilla.

There are other search engines that may be interested in something like this. While strange bedfellows for geopolitical reasons, I could see Yandex or Baidu doing a default search deal in order to break into the English speaking market a little more.