One the one hand, Chazal teach us that appointing a king is one of the 613 mitzvos, yet on the other hand, the Torah presents the idea of appointing a king as a response to the demands of the people, "V'amarta asima alay melech…", implying that where there is no demand, there is no requirement to appoint a king. The Netziv explains that there is no contradiction. The mitzvah of appointing a king does not require imposing a monarchy willy-nilly. There is a built-in condition to the mitzvah that calls it into effect only should the people demand a monarchy. In some countries a democracy may be the preferred form of government, in other countries other systems may work. However, should the people demand a king, the Sanhedrin is commanded to then act on that demand and put in place a monarchy which conforms to the principles set down in our parsha.
[This post has been updated - the idea of an issur aseh which I mentioned originally is in fact rejected by the Netziv.]