Why enchant lower tiers of armor? Maybe the effort required to enchant Chain is lower than the effort required to enchant Banded. Maybe the civilization that enchanted the armor never developed Banded armor.
Heavy Armor proficiency is an advantage, so heavier classes of armor should be better than lighter classes of armor, at least at the job of protection.
Within the class, the various types of armor are not interchangeable in their use. In 3e, armor prices where for the most part low and flat. In 4e, base armor type prices where even flatter (with plate at a cheap 50 gp), where your access to better armor was controlled by armor-specific proficiencies. In AD&D, field and full plate existed, which where mundane armor that was expensive enough that lower end magical armor could be cheaper than it.
In the playtest packet, higher end mundane armor is basically "magical" in quality. The top tier of each armor type uses fantasy materials to provide its protection. Saying that it should be worse, somehow, than lower armor in the same type would be like saying that +2 armor should have a disadvantage to make getting +1 armor instead tempting.
Now, the lighter armors are probably too cheap in the playtest packet, resulting in characters being able to bump themselves up a step on the lower-tier armor (ie, the "+1 leather" equivalent is cheaper or compatible in price to unenchanted heavy armor in the playtest, for a 2e/3e/4e explanation). That may not have been wise.
Really, having a 0/1/2 yard per round reduction in your speed, together with stealth disadvantage for medium/heavy armor, is probably enough. Someone in light armor with max-character dex should have the same AC as someone in heavy armor, and the +2 movement and lack of stealth disadvantage should be sufficient payoff you get for having your high dex.
Heavy Armor proficiency is an advantage, so heavier classes of armor should be better than lighter classes of armor, at least at the job of protection.
Within the class, the various types of armor are not interchangeable in their use. In 3e, armor prices where for the most part low and flat. In 4e, base armor type prices where even flatter (with plate at a cheap 50 gp), where your access to better armor was controlled by armor-specific proficiencies. In AD&D, field and full plate existed, which where mundane armor that was expensive enough that lower end magical armor could be cheaper than it.
In the playtest packet, higher end mundane armor is basically "magical" in quality. The top tier of each armor type uses fantasy materials to provide its protection. Saying that it should be worse, somehow, than lower armor in the same type would be like saying that +2 armor should have a disadvantage to make getting +1 armor instead tempting.
Now, the lighter armors are probably too cheap in the playtest packet, resulting in characters being able to bump themselves up a step on the lower-tier armor (ie, the "+1 leather" equivalent is cheaper or compatible in price to unenchanted heavy armor in the playtest, for a 2e/3e/4e explanation). That may not have been wise.
Really, having a 0/1/2 yard per round reduction in your speed, together with stealth disadvantage for medium/heavy armor, is probably enough. Someone in light armor with max-character dex should have the same AC as someone in heavy armor, and the +2 movement and lack of stealth disadvantage should be sufficient payoff you get for having your high dex.