Beyond the heat, I think familiarity. For some reason, division games are just difficult for every team - as a whole. You play them twice a year, every season. There are playoff implications, pride, animosity. Professionals can say games are all important, but human nature is a factor.
I've lived, worked and vacationed in heat like Miami. Sections of New Orleans are 7 feet below sea level and talk about humidity and heat. When you reside in a climate like this, you gradually acclimate to the conditions as the year progresses and becomes warmer. It may not happen the first year either. The same applies to extreme cold. When Miami faced KC a couple of years ago in the postseason the Dolphins were in literal shock at the subzero temps. Tyrek Hill who was a terror in the same cold when he was a Chief, wasn't the same player as a Dolphin returning to Missouri in January.
I moved from a hot/humid city back to NE and the very first winter back was rough.