Objection 1. It would seem that temperance is not a cardinal virtue. For the good of moral virtue depends on reason. But temperance is about those things that are furthest removed from reason, namely about pleasures common to us and the lower animals, as stated in Ethic. iii, 10. Therefore temperance, seemingly, is not a principal virtue.
Objection 2. Further, the greater the impetus the more difficult is it to control. Now anger, which is controlled by meekness, seems to be more impetuous than desire, which is controlled by temperance. For it is written (Proverbs 27:4): "Anger hath no mercy, nor fury when it breaketh forth; and who can bear the violence [impetum] of one provoked?" Therefore meekness is a principal virtue rather than temperance.
Objection 3. Further, hope as a movement of the soul takes precedence of desire and concupiscence, as stated above (I-II, 25, 4). But humility controls the presumption of immoderate hope. Therefore, seemingly, humility is a principal virtue rather than temperance which controls concupiscence.
On the contrary, Gregory reckons temperance among the principal virtues (Moral. ii, 49).
I answer that, As stated above (123, 11; 61, 3), a principal or cardinal virtue is so called because it has a foremost claim to praise on account of one of those things that are requisite for the notion of virtue in general. Now moderation, which is requisite in every virtue, deserves praise principally in pleasures of touch, with which temperance is concerned, both because these pleasures are most natural to us, so that it is more difficult to abstain from them, and to control the desire for them, and because their objects are more necessary to the present life, as stated above (Article 4). For this reason temperance is reckoned a principal or cardinal virtue.
Reply to Objection 1. The longer the range of its operation, the greater is the agent's power [virtus] shown to be: wherefore the very fact that the reason is able to moderate desires and pleasures that are furthest removed from it, proves the greatness of reason's power. This is how temperance comes to be a principal virtue.
Reply to Objection 2. The impetuousness of anger is caused by an accident, for instance, a painful hurt; wherefore it soon passes, although its impetus be great. On the other hand, the impetuousness of the desire for pleasures of touch proceeds from a natural cause, wherefore it is more lasting and more general, and consequently its control regards a more principal virtue.
Reply to Objection 3. The object of hope is higher than the object of desire, wherefore hope is accounted the principal passion in the irascible. But the objects of desires and pleasures of touch move the appetite with greater force, since they are more natural. Therefore temperance, which appoints the mean in such things, is a principal virtue.
Source: Summa Theologica (New Advent)