Evaporative cooling works best in hot, dry climates. Mississippi is pretty much a hot, humid climate.
Evaporative cooling works because it takes a lot of energy to turn liquid water into water vapor. One BTU (British Thermal Unit) is the energy needed to raise the temperature of one pound of water one degree F. Evaporating the same pound of water takes about 1000 BTU. If you have a 100 lbs of water and 1 lb evaporates out, the remaining 99 lbs will be 10 F cooler.
So what is the limiting factor? Dew point of the air, or the wet bulb temperature. Air at any given temperature can only hold so much water before it becomes saturated. In Arizona it's a long way from ambient conditions to saturation, so lots of water can evaporate, which makes for lots of cooling. In Mississippi, OTOH, the distance is much shorter.
One thing that will help in humid environments is to set up a fan blowing at the cooler. This blows away more saturated air, replacing it with less saturated air. This evaporative cooling is by far the most common type of cooling used in industry, power generation, and cooling of large buildings.