Why can't Armorer make armor for himself?

Prior to 1.19 ARM was able to make a number of armor pieces they could wear.
Now all we get is the Mesail.
Outside of Vintage, there is no armor we craft that is usable to us for the body, feet, waist, and hands.

If you have no plans to fix this, could we at least know why this decision was made?

I think its pretty much a given that recipes are all screwed up at the moment. Many were removed with no notice, many changed, not many added. Unfortunately recipes are a work in progress. It seems strange to be playing the game half-finished (oh wait maybe not) but the full list of recipes will come in 1.2 or later and its just not worth getting worked up about it until the final list is released.

I don't think you'll see a dramatic change in the armor and weapon crafting recipes. There's an item for every slot for every level right now. Armor for crafting, gathering and casters is from weaver, helms/belts/feet are from leather worker or weaver, and of course tools and weapons are from the same places as before. There's no holes in the gear, but you will have to dye to get the right stats. Cooking, alchemy and jewelry will get dramatic changes in 1.20, but the rest will likely just have the dated gear removed, and perhaps the class requirements adjusted.

Because pounding away on armor while wearing armor would get really hot? You can waer armor but just wont get benefit from it because you'll be sweating.. just a guess.