Google decided it would be more beneficial (to tyhem, or to the cause) by just raising awareness and still providing the useful information the user wants.
Blackout -> User sees it once and goes to SearchEngineX, forgetting all about it.
Google's Way -> Raise awareness each time the user sees it as they won't go to a different Search Engine.
A full blackout works for wikipedia, because people probably don't know about any alternative - especially with their pages dominating search results. But with Google, most people know of Bing or Yahoo at the very least.
Even Wikipedia's blackout is not full, and quite easy to go around if you know magic tricks like disabling JavaScript or CSS in the browser. While commercial damage is likely a very important consideration to Google, another reason might be that shutting down Google completely would most likely result in damage to human health and/or loss of lives.
Blackout -> User sees it once and goes to SearchEngineX, forgetting all about it. Google's Way -> Raise awareness each time the user sees it as they won't go to a different Search Engine.
A full blackout works for wikipedia, because people probably don't know about any alternative - especially with their pages dominating search results. But with Google, most people know of Bing or Yahoo at the very least.