Franklin's goal here is not just moral behavior but moral perfection. Why do you think so few people today believe in such perfection?

I would echo the sentiments previously discussed.  Might I also suggest that perfection has lost its sheen, in part, because we understand more of the frailties of human beings which makes it impossible, to a great extent.  For example, while Franklin was speaking from a supposed moral perfectionist position, his own social setting allowed slavery, subjugation of women, and denial of voice from the lower class.  We also know that Franklin himself engaged in activities that we now would deem as far from perfect.  Part of the reason that we have lost a sense of perfection is because with the more information and understanding we have gained, such intense scrutiny has allowed us to see faults and shortcomings in such a light that we have discredited those who have suggested that perfection is a reality.  Analyzing Franklin in this light translates into doing the same with others.  Those who have professed to possess a vision of perfection have articulated this in the face of striving for a comedic ends, unable or unwilling to acknowledge its inevitable tragedy.  We have begun to understand that while perfection might not be the ultimate end, "getting it right" could be a more worthy goal.  It is for this reason that Franklin, himself, and the other framers listed as the first goal of the Constitution to "form a more perfect union," implying that democracy is an experiment where perfection is impossible, but "getting it more right" is in ofitself a more worthwhile pursuit.