Why aren't any jet fuels heavier than Jet A/A-1/JP-8 in common use?
As @john K said, fuel needs to be liquid at cruising altitude. The less volatile a hydrocarbon is at a temperature, the quicker it will stop flowing as the temperature drops. 

Additionally, the plane is not necessarily landing in the same hot climate it is taking off from. You need a fuel that is stable across the range of temperatures experienced while operating.

This post addresses the specific behavior of different fuels in great depth.