Well, the simple reason is probably just that more people use google

easy, that's becauz when most peole want know something nowasdays , they simply google it.

Another reason is that Google search has been made available:
1. As the first choice on FireFox search box. Yahoo is also a drop down option but I wonder which percentage of users actually purposely stroll down to use Yahoo.
2. on major mobile devices - I have both a Blackberry and iTouch and I have a Google application for both. I did not find a Yahoo application so of course I will choose Google.

Thus, Google cast a far wider net than the other two search engines which may explain why more people use Google. Hardly anyone says: "Yahoo it!"

Palm devices use Yahoo and Windows Mobile Devices use MSN/Live/Bing

This is because Google is first choice for most of the people.

http://l.yimg.com/a/i/ww/beta/y3.gif

http://www.google.com/intl/xx-elmer_ALL/images/logo.gif

Which logo catches your eye the most?

It all depends on the type of message the logo is trying to evoke. The red in Yahoo evokes power meanwhile the rainbow like colors and the mostly blue hues in Google evokes high levels of trust. Look at the logos for major online networking sites, there are all blue - Myspace, LinkedIn, and Facebook.

I think because Google is the most trusted search engine in the whole wide world.

HAHA your title and your content are actually two completely different questions
your title ask about google--i guess just becauz it si the biggest
and your thread is about yahoo and msn SEO