For the record, Yahoo search is now powered by the Bing search engine, so your actual question is why Google is better than Bing. Possibly this is just semantic nitpicking, but I think a lot more people have opinions on Bing vs. Google than on Yahoo vs. Google. I’ll be honest, I didn’t even know until you asked this question that Yahoo even still had a usable search engine. I thought they’d switched entirely over to being a portal site a long time ago.
Here’s a fairly detailed comparison from PC World about the differences between Google and Bing.