Because Google is JUNK. 90% of the time I search for a specific question in Google or Hakia (yes you Hakia fans it is not that great either) it is totally irrelevant to what I am looking for. Or it just doesn't match what I am looking for by an inch.
There are many other reasons. Perhaps they find Wikianswers more fun to use, or easier, or they want a good consise summary of the answer without needing to read lots of text.