Why do Marxists always blame someone else?

Karl Marx kicked off the blame game by blasting the communards of the Paris Commune because they didn't seize the banks in Paris, leading to their demise. Stalin blamed "counterrevolutionaries and Zionist Jews" for the shortcomings of the USSR. Mao blamed old school thinkers and then launched what he called the "100 Flowers" campain. Then he executed everyone who had ideas he didn't like. Pol Pot executed everyone except for children under 5, because he thought they were not tainted. Castro always blames the CIA. What is it with Marxists that its always someone elses fault when the economy tanks? I hear the Obama is in Texas right now, blaming Bush for everything.