The Ramban (beg. Parshas Shoftim) cites the Rambam as holding that the mitzvah to appoint judges applies only in Eretz Yisrael. Ramban challenges this view based on a braysa (Makkos 7) that says that the mitzvah does apply in chutz la'aretz, albeit in a modified form: in Eretz Yisrael there is a mitzvah to appoint judges in every city and every district; outside Eretz Yisrael it suffices to have district judges alone.
Achronim dismiss the Ramban's question because the text we have of the Rambam matches the braysa exactly. However, the is reason to believe some other text of the Rambam (or a similar view in Rishonim) does exist in line with what Ramban quotes. The Sefer haChinuch, who usually echoes the Rambam, does write that the mitzvah of appointing judges applies only in Eretz Yisrael. Why should this be?
Even Ramban's position that there is an identical mitzvah of minuy in chutza la'aretz begs the question of why there is a distinction between Eretz Yisrael, where judges are needed in every city, and chutz la'aretz, where district judges suffice.
There may be two distinct components to the mitzvah of establishing courts: 1) As an end in itself, to fulfill the mitzvah of minuy dayanim; 2) As a necessary means to preserving a just and lawful society.
The mitzvah of minuy dayanim as an end in itself applies only in Eretz Yisrael. This is why the Rambam/Chinuch write that the mitzvah applies only in Eretz Yisrael and why every city must establish a court. However, there is still a need for batei din in chutz la'aretz to enforce justice on an ad hoc basis, as a means to preserving social order. Since the courts in chutz la'aretz are just a means to this end, district courts alone suffice. (See Avi Ezri, Hil. Melachim ch. 9 who applies this distinction to the mitzvah of dinim for ben noach.)