Because either one of them could hold the office of president.

Example one: McCain passes away in office thenshe becomes president. In many conservatives eyes, it wouldn't be a bad thing as she is more consevative then McCain and is more then likely to go consevative.

Example two: If Obama passes away, then Biden becomes president. Once again, I'm sure that many conservatives would rather see Biden then Obama anyway for he has a history of supporting the Iraq War.

Like or not, their past is relevant.