The answer is right on the 警視庁 (metropolitan police department)'s website.
Basically it says that it is common to use foreign words as-is if there is no similar cultural counterpart, using sumo and kabuki as examples. Koban is an unfamiliar idea in most cultures, so that's why they decided to go with using "koban" as is. The koban system has been introduced in the west.
They also make an effort to reinforce this on their English website.
While "police box" is often used as the closest translation, police boxes are actually very different from koban, in both size and purpose. The MPD probably wants to make this distinction. Also, not every Japanese resident can think up the words "police box" and this can hinder communication with a non-Japanese speaker; they would most certainly be able to point a lost tourist to the nearest "koban".