Because Machiavelli is fundamental to good politics.
When we talk about Machiavelli, we tend to remember the bad politicians: those who lie and plot devices to conquer and maintain power, in which they harm the population. In Vatter's explanation, it is clear how Machiavelli's teachings are equally - or even more - relevant in the conduct of the good politician. After all, people's freedom from other peoples depends on it.
Internally, it depends on the preservation of the people's freedom against the wishes of the elites who want to dominate them. After all, in every society there will be those who seek to dominate others. If there is no maximum authority that guarantees the interest of the majority (of the people) in not being dominated, it will be in the hands of its dominators. The abuses of the rich and powerful are a constant risk, and only a power greater than theirs, that of the State, can prevent them.
The desire for domination continues to exist. But, thanks to a system of laws and coercion, it is now channeled no longer towards domination by arms, but towards private ends: accumulation of wealth in the market, for example. In order to institute this system of respected rights, the State must necessarily violate these rights in an original act of creating laws, which depends on violence above all.
Machiavelli's world is no longer the medieval world in which a metaphysical order determined human good, and the function of political authority was to direct men to virtue. Now, there is no more order; only desires, and the role of the State (that owner of violence) is to ensure that each one can pursue his interests without hurting other citizens. In other words, Machiavelli's vision is our current conception of society.