Maybe he knew that that war was started on a lie. Why go fight a war that your government got you involved with a lie. The thousands / millions that died there should be looking for compensation from the US or at least an apology.
Also, as Ali said, he had no desire to 1) kill thousands of people who hadn't done anything to him 2) defend a country where - after winning a gold medal - he was called n@@ger and wasn't good enough to be served at a lunch counter because his skin was the wrong color.