Why can't we boil down the three fonts of morality to two?
The Catechism of the Catholic Church states that the morality of human acts depends on three things:
I. THE SOURCES OF MORALITY
1750 The morality of human acts depends on:
The object, the intention, and the circumstances make up the "sources," or constitutive elements, of the morality of human acts.
1751 The object chosen is a good toward which the will deliberately directs itself. It is the matter of a human act. The object chosen morally specifies the act of the will, insofar as reason recognizes and judges it to be or not to be in conformity with the true good. Objective norms of morality express the rational order of good and evil, attested to by conscience.
1752 In contrast to the object, the intention resides in the acting subject. Because it lies at the voluntary source of an action and determines it by its end, intention is an element essential to the moral evaluation of an action. The end is the first goal of the intention and indicates the purpose pursued in the action. The intention is a movement of the will toward the end: it is concerned with the goal of the activity. It aims at the good anticipated from the action undertaken. Intention is not limited to directing individual actions, but can guide several actions toward one and the same purpose; it can orient one's whole life toward its ultimate end. For example, a service done with the end of helping one's neighbor can at the same time be inspired by the love of God as the ultimate end of all our actions. One and the same action can also be inspired by several intentions, such as performing a service in order to obtain a favor or to boast about it.
1753 A good intention (for example, that of helping one's neighbor) does not make behavior that is intrinsically disordered, such as lying and calumny, good or just. The end does not justify the means. Thus the condemnation of an innocent person cannot be justified as a legitimate means of saving the nation. On the other hand, an added bad intention (such as vainglory) makes an act evil that, in and of itself, can be good (such as almsgiving).
1754 The circumstances, including the consequences, are secondary elements of a moral act. They contribute to increasing or diminishing the moral goodness or evil of human acts (for example, the amount of a theft). They can also diminish or increase the agent's responsibility (such as acting out of a fear of death). Circumstances of themselves cannot change the moral quality of acts themselves; they can make neither good nor right an action that is in itself evil.
Thus as we learn from the Catechism that the circumstances of our actions will determine whether or not we shall merit before God for our good moral actions or be more guilty or culpable before God in regards to our immoral or sinful actions.
The circumstances of our moral actions are the variables by which help to determine how guilty one is in regards to sinful acts. Even the courts of law admit that some crime may be committed by some individual, yet a prosecutor may be inclined to ask the courts for a lesser sentence (probation in lieu of jail time) due to extenuating circumstances.
The opposite is equally true for gaining merit before God, for doing good.
You can not take the circumstances out of the picture when describing the morality of human actions as they form a intricate part of morality.
St. Thomas Aquinas explains in his Summa Theologiae why circumstances are so necessary in regards to human acts:
Whether theologians should take note of the circumstances of human acts?
Circumstances come under the consideration of the theologian, for a threefold reason. First, because the theologian considers human acts, inasmuch as man is thereby directed to Happiness. Now, everything that is directed to an end should be proportionate to that end. But acts are made proportionate to an end by means of a certain commensurateness, which results from the due circumstances. Hence the theologian has to consider the circumstances. Secondly, because the theologian considers human acts according as they are found to be good or evil, better or worse: and this diversity depends on circumstances, as we shall see further on (I-II:18:10-11; I-II:73:7). Thirdly, because the theologian considers human acts under the aspect of merit and demerit, which is proper to human acts; and for this it is requisite that they be voluntary. Now a human act is deemed to be voluntary or involuntary, according to knowledge or ignorance of circumstances, as stated above (I-II:6:8). Therefore the theologian has to consider circumstances.