Because McCain decided to make an issue out of it? Probably.
McCain went and made a big whoop, enlisting a truth squad. Now, he just seems like an old man, trying to command respect. Why can't we respect his service but question his experience?
Clark is right. Why is McCain perceived as more capable to be commander in chief? Because he was a war hero is all I know. I'd like to hear how being a war hero qualifies him to be commander in chief.
I'd rather have a leader who is less inclined to solve problems with military answers. If McCain was president, Bush would simply hand him off the war and there we would be for years, maybe 100.
update: Obama rejected Clark's statements. Apparently, military service is to McCain as race is to Obama (that's courtesy of ABC news).