Bcoz Google hold 70-80% market in the world of internet, and easy user interface so comfortable for user.
Google is a household name. My 70 year old boss comes up to me and asks me to "Google Something." Not search for something, it's google something. Google has a very good market and brand that has spread like a wild fire.
I use google for hosting my domains on the Adsense for domains. Pos-t-vac(dot)com does pretty well. However, I may be switching another domain I have to yahoo or Sedo for parking. I get 100+ uniques a day on pos-t-vac(dot)org and only have a 5% ctr with a cpc of about $0.05
Google Is great guys it shows result very simple. google is a big search engine rest of all other, more Powerful then other, more popular the other. every one ask about it even my client demand only google ranking so i use it mostly.