Do a search about coop fires. Red lamps being used on adult birds is the absolute main cause.
Size of coop may not allow birds to choose to be under it or away from it causing overheating, burned feathers or the birds to crash into it causing breakage or fires.
Properly acclimated birds have grown new feathers in preparation for cold weather. Heating the coop can cause birds to not grow the needed feathers and a power outage is most likely going to happen in the absolute worst weather. It is a shock to their systems and can be deadly.
In winter birds need really good and proper ventilation to allow moist air caused by their breathing and droppings to escape. Proper ventilation makes heating moot/useless.
Those people living in extremes where temps are -20F or colder for weeks on end may need to heat their coops. The rest of us not so much.