The main difference there between Phoenix and DC is the humidity. I can understand for retirees, but isn't humid air what keeps the ball in the park in Denver? 
Contrary to popular belief, balls fly further in humid air than they do in dry air.
Assuming no breeze, factors affecting the distance of balls hit in the air, in descending order of importance:
1) Altitude - higher means thinner air, longer flight.
2) Heat - hotter air expands, fewer molecules in the same space, therefore hotter is better
3) Humidity - water molecules take up more space, causes air to be LESS dense than when dry, hence balls fly further.