I think we'll find that the statistics weren't kept for older questions from quizzes and starting to track % some time after their release would not have been a true value of the % correct and so it is not shown (but I suspect there is a value stored for quiz assembly purposes, we just won't see it). I will qualify that as opinion and not based on anything but reading previous postings in other threads about it under other thread titles.
"...Tomorrow's come a long way to help you."
Tim Davis 'Your Saving Grace' Steve Miller Band (1969)
"...Yesterday's at least a mile back."
Dale Peters 'Dreaming in the Country' James Gang (1971)