That's because they are penalizing people who are trying to manipulate search results in an obvious manner. Excessive use of keywords and hidden text are outdated methods used by less knowledgeable site owners who still believe that these methods work.
Originally Posted by ChillingBreeze
In a 400 to 500 word article, it is a good idea to mention the keyword theme for the page once or twice in the text, but packing keywords into the text is bad idea. You should always write content for your users, but do not completely ignore what search engines are looking for.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin