I use both, more so google. But Google yields better results for me most of the time. But as others have said, the answer is obvious, bing just came to late to the game.
Once people form habits they are hell to break (such as most people using google). In order for yahoo to compete or gain ground they must give the customer a compelling reason to switch. Up to this point they have not been able to accomplish this. When i was younger i thought Google was the internet!
Bing as far as I can remember was later created. And as we know the old provider is much better new. Though the new fashion.
I find that Google has better quality results than bing the the majority of searches that I have made. Bing is at least trying though.