Horse latitudes or Subtropical High are subtropic latitudes between 30 and 35 degrees both north and south. This region, under a ridge of high pressure called the subtropical high, is an area which receives little precipitation and has variable winds mixed with calm.
So basically, because these are belts of high pressure around the earth, the movement of winds will be out of these areas, leaving them calm. Air flow will move away from these high pressure zones, and divergent by definition means, "tending to move apart in different directions."
ANSWER:
So because of the movement of winds away from these latitudes, they are called divergent latitudes