Dave Winer :

Ironically, Knol probably would have fared better if instead of having the appearance of Google tilting the table in its favor, search engine-wise, they had put something in its robots.txt file that kept the Google crawler away, so that the opportunists would have stayed away too. That would have given them some time out of the spotlight to build up some real momentum, giving it a chance to compete with Wikipedia. Not sure what Wikia could have done, the idea seemed doomed from the start, because search isn't like a Wiki, and human-authored search results are something of a contradiction.