Chain stay mounted brakes became popular in the late 1980s. This frame is much stronger and stiffer in this area than the traditional seat stay mounting area. Consequently, when the brakes are applied there is less reaction from stays to flex outward and decrease braking effectiveness. While the improvement was tangible, the location made it much harder to set-up and adjust the brakes. It was also prone to mud build-up. Their popularity waned waned after a few years.

Edit: Are you sure this isn't more like 1987-1989? 1993 is very late for a chain stay mounted brake on a high end ATB. Pics? Serial number?