I think Goombah has it right. Hubris was at work in that scene. McCluskey never, ever expected that some "guinea hood" would attack a police captain. He had one or two seconds to react and possibly save himself. But he probably spent them either expecting Michael to surrender to him or trying to comprehend what just happened. Fatal mistake.
Underestimation plays a key role throughout the Trilogy. Vito said, famously, "Women and children can afford to be careless. Not men." Look at the consequences of underestimation, which is a major form of carelessness. Vito underestimated Sollozzo. Sollozzo and McCluskey underestimated Michael. Sonny underestimated Carlo. Barzini and Tessio underestimated Michael. Roth and Michael underestimated each other several times. And on and on...