The answer can only be: because the ongoing history of Israel, especially that of its period as a nation-state, was not regarded as a time that lent Israel its identity. The final redactors of the Torah were of the opinion that what Israel was in its innermost self, what constituted it, what it was for, was revealed not under David and Solomon but under Moses, and, more precisely, it revealed itself in Israel’s liberation from Egypt and the covenant with its God at Sinai.17
It only becomes clear what that ultimately means when we keep in mind that, at the time when the Torah was created as a unit, the royal period was already in the past. People could look back at that period in its entirety, and in the eyes of the final redactors of the Torah, who were seeking God’s true will in and for history, the royal period was not only an unlucky era but a theological catastrophe. That period could in no way be one that created identity. It could only be a time of warning against going that way again. Therefore it was not included in Israel’s basic text. Instead, the identity of the people of God was sought in Israel’s early period, in the time of the “patriarchs,” the time when a covenant was made with God, the time when Israel was still on its way, the time of its testing in the wilderness.
Consequently, the Torah shows little interest in an earthly king. The covenant God forges with Israel makes every worldly king a spectator, and so the legal materials in the Torah for the most part say nothing about a king or a state. If we sift through the concrete law in the Torah we see immediately that the king plays an insignificant role; in most of the law collections he does not appear at all. There are cultic laws, social laws, family law, but scarcely any law applying to political institutions. The only partial exception is Deuteronomy 17:14-20, a law concerning the king:18
When you have come into the land that the LORD your God is giving you, and have taken possession of it and settled in it, and you say, “I will set a king over me, like all the nations that are around me,” you may indeed set over you a king whom the LORD your God will choose. One of your own community you may set as king over you; you are not permitted to put a foreigner over you, who is not of your own community.
Even so, he must not acquire many horses for himself, or return the people to Egypt in order to acquire more horses, since the LORD has said to you, “You must never return that way again.”
And he must not acquire many wives for himself, or else his heart will turn away; also silver and gold he must not acquire in great quantity for himself. When he has taken the throne of his kingdom, he shall have a copy of this law written for him in the presence of the levitical priests. It shall remain with him and he shall read in it all the days of his life, so that he may learn to fear the LORD his God, diligently observing all the words of this law and these statutes, neither exalting himself above other members of the community nor turning aside from the commandment, either to the right or to the left, so that he and his descendants may reign long over his kingdom in Israel.
This text shows that Israel need not have a king at all. That in itself is something revolutionary against the horizon of ancient Near Eastern societies. But if Israel really wants a king it should install him itself—and in terms of constitutional law that means there is to be no absolutism but only a constitutional monarchy. The constitutional nature of the idea is also apparent in the fact that the Torah is above the king. He is to read it daily and keep it with him at all times.
Besides, the king’s power is limited: he is not to lead the army in war (cf. Deut 20:9, where the king does not appear). He has nothing to do with the observance of the laws (cf. Deut 17:8-13, where the king is also absent). And he is not the final instance for appeal (Deut 19:17). If we look at the matter closely we find that there is to be a division of authority in Israel: independent judges, independent priests, a prophet from time to time, always one chosen by God (Deut 18:18). The king is also to be distinguished from the other Oriental potentates in that he keeps his chariots, his harem, and his treasury within limits. Thus in the projected constitution in Deuteronomy 17 he plays only a marginal role. He is not central. Compared with the usual custom in the ancient Near East, his status was entirely relative. To put it in a nutshell: his main task was to study the Torah every day.