I think the biggest factor will be web browser bundling and name recognition.  Yahoo & Google are household names.  Bing has a way to go in that area.  Although it has the benefit of being bundled with IE8 and Windows 7.  Safari, Chrome, and Firefox generally default to using Google as their search engines.  Where IE8 defaults to Bing.  Yahoo is only gaining marketshare through browser plugins and widgets and is slowly loosing their crowd.  Here are some interesting stats from a SearchInsider article forwarded from a colleague and friend.

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