The temperature generally swings something like 20F during the course of a 24 hour day. At night, I am quite content to have the windows open. During the day, I'm not around so why bother cooling the house. Then when I get home from work, it'll take the rest of the evening to cool off the house, at which point I might as well have just opened the windows and let everything cool off naturally.

Contrast that to the winter, where you can quite easily have a whole range of temperatures throughout the day based on being home or away and dropping the temp down at night while you sleep.

Heat pumps I'd guess work quite well over the same delta range that summer AC usually operates in (30F probably, even on the hottest days).

Beyond that, natural gas or propane is going to be much more cost effective.