Franklin, as a man of the Enlightenment, believes fervently in the idea that man can become a perfect and rational being by adopting certain rules by which to live his life:
It was about this time that I conceived the bold and arduous project of arriving at moral perfection. I wished to live without committing any fault at any time. . . .
This goal, of course, no matter how admirable may seem ambitious--perhaps impossible--to us in the 21stC., but men in the middle of the 18thC. absolutely believed that becoming morally perfect was not only possible but a positive duty.
When Franklin creates his list of moral virtues, he defines temperance, which appears first, as eating only until one is no longer hungry (and no more) and drinking only to satisfy thirst, not to become drunk. In the introduction to the list of virtues, however, Franklin comments that temperance often includes
. . . moderating every other pleasure, appetite, inclination, or passion, bodily or mental. . . .
Franklin decides, for purposes of clarity, to expand the list of virtues to create a clearer list of virtues and their accompanying rules, and therefore to limit temperance to food and drink. This broader view of temperance, which extends to "every other passion, appetite, inclination," is consistent with the Enlightenment's view of the value of regulation in all aspects of life--physical, moral, spiritual.
For Franklin, then, temperance came first primarily because if one can control powerful bodily urges to eat and drink to excess, the control of other virtues--resolution, frugality, industry, among others--becomes more easily accomplished. In other words, if men can exercise one virtue, they can exercise all virtues.
We debate whether Franklin took his own advice completely over the course of his life, but the importance of the list of virtues is that men in the 18thC. believe wholeheartedly that such virtues are attainable.