Me, I'd blame the Reforms.

Heavy armor is a bane in cold weather as well - when freezing, it burns like fire. Metal conducts heat. Boiled leather, especially reinforced by metal or lacquered wood, is more friendly in a northern clime.

It's a matter of training, mind. Personally, I can't even keep a shield up for 20 minutes. Marching in full armor with a pack... that's waaay beyond me. And yet, I don't train with either... which means I'd be a very poor legionary in the days of the Republic.