Confirmation and recency biases along with basic demographics?
IOW, for someone to give a rating other than "Don't Know" to say Carter or Ford, they would have likely had to be at least in their teens to have an opinion that allows them to compare/rank against a more recent POTUS.
My dad will be 79 soon. I doubt he would really care to have an opinion on FDR or Truman compared to say Kennedy, Johnson, Nixon.