The Tao of D&D comes a table that will restrict the use of the warmer armors in different temperature ranges. There may be slight adjustments for people native to this area, but even we eschew very heavy armor to avoid heat-related ailments.
Time to strip down to the essentials, unless it's time to fight! Right, Amaya?
Seriously, though, some of it is also a matter of available materials. Perhaps the iron ore is difficult to get at, or at least in quantities that would make chain mail or plate mail likely to be produced (and maintained) on a larger scale. Or perhaps the powers that be don't look kindly on anyone manufacturing large amounts of armor?
Speaking of maintenance, being near the sea (and all that salt water) might make fighting against rust a real pain. How quickly does uncared for metal armor rust? Like those pieces of armor found in dungeons that may or may not have had blood spilled on them?
Another concern is the craftsmanship -- perhaps the skill is there for creating swords and other weapons, but I'm sure that it must be a pain to 'start over' when you've made a mistake with your armor crafting for chain mail or plate mail...