I would think it has something to do with safety. I remember seeing an Air Crash Investigation on National Geographic which was partly caused by sitting on the tarmac with the aircon running for a long time before taking off.
That will heat up the aircon system significantly and apparently that is (at least in that plane) sitting right below one of the fuel tanks.

I am just speculating though, but I'm sure someone knows the exact reason and will pass by and tell us