Because the Democrats have eschewed the type of negative campaigning that is commonplace among Republicans, John McCain's abysmal naval record is little known. I think it is relevant to know that he finished 894th in a class of 899 at the Naval Academy, and he crashed several jet fighters during his flying career. Anyone else would have lost his wings, but because of McCain's family connections, he repeatedly was given special treatment. Further, McCain's womanizing and temper tantrums are legendary among those who knew him in his early days.
This helps explain why McCain has run such a pathetic ampaign--he just doesn't have the smarts and doesn't have the judgment.