1.) Relative power. It has the highest nonmagical AC you can acquire without a shield (or feat). So while it is "nonmagical" and purchasable, its as powerful as say, +1 half-plate or +1 splint.
Rare plate is the most punitive for a fighter, where other hybrid classes have spells or other defenses to compensate. I guess to equalize things then make all armor rare or harder to make, and make defensive spells harder to acquire; like mage armor.
Technically it is only punitive if the player was dumb enough to fall for the PHB's out-right lie that a Strength-based fighter was something that was remotely meant to be played unless one was playing a human and took the Great Weapons-Master or the Polearm Master feat or a Half-Orc intending to take that feat.
In any other case, it isn't even remotely competitive with the Dex-based Fighter regardless if you go dual-weapon or weapon and shield, losing in both ranged and melee. The Dex-based Fighter can buy all the equipment needed to be superior from level 1 and really never needs to switch any of it out until they find magical versions.
given the AL loot system they are actually better if doing just that rather than taking the +1 splint.Seeing as the thread got necroed - there is one significant disadvantage to regular plate vs +1 splint: oozes and rust monsters. Consider how many AL modules had one form of an ooze or another and you'll see how a splint mail +1 is a better equipment option than plate. I've seen several cases of macho front liners run into corners with their skirts up soon as a black ooze made an appearance leaving the job of tanking/killing/kiting to more mobile and ranged classes.
Also gold earned picks up drastically in Tier2 AL. So even if you lose your plate to every single pudding that shows up, it ceases to be an issue soon.Going further with AL examples - you could potentially get plate (or for that matter splint +1) as early as level 5 although armour distribution in AL is very questionable. You can't loot plate from an outlaw (that could potentially be taken alive) in a 1-4 module but you can loot it from a corpse in a 5-10 module. In the same vein - you can loot splint mail from an enemy that can be taken alive or dead in an earlier 1-4 module so the whole 'does it have to be fitted/undamaged debate is nonsensical in this context.
One thing to bear in mind about magic item rarity is that it doesn't tell the whole story.That's a bit like claiming that chain mail (part of the starting gear for fighters) is as powerful as Bracers of Defense. I'm not seeing it. And it's quite odd that you consider nonmagical plate armor to be rarer and more expensive than mithril/adamantine plate armor. I think you are inferring the wrong things from the 500 gp "construction price" for uncommon magic items. But, YMMV.
From 1974 to 1985 there was no "full plate" armor in D&D. "Plate mail" only gave an AC of 3. According to the conversion document, this was equivalent of AC 16 in 5e, what you get from breastplate plus the allowable Dex bonus. Personally, since I like to emulate an old school style and feature a medieval level of technology in my campaigns, the comparative rarity of higher qualities of armor suits me just fine. In fact, I use half plate and plate to represent a higher level of technology than is available to the average person, and splint for an exotic technology that is just as rare.
Full Plate Armor (AC 1)
Description: Full plate armor is the best armor a warrior can buy, both in appearance and protection. The perfectly-fitted interlocking plates are specially angled to deflect arrows and blows, and the entire suit is carefully adorned with rich engraving and embossed detail.
Campaign Use: Suits of full plate armor are as rare as powerful magical items in most fantasy campaigns. Magical sets of full plate are artifacts to be treasured and hidden away, the objects of glorious quests.
In most campaigns, the number of sets of full plate armor can be counted as easily as the numbers of crown knights who owe their allegiance to the king. In many kingdoms, it is a crime to possess a set of full plate armor without royal permission, as a wise king keeps any armorer capable of such craftsmanship at his beck and call.
Full plate armor is one of the greatest gifts a great lord can bestow upon his followers. It is a prize as coveted for the status it confers as its monetary value. A suit of full plate armor will often be a gift presented to great knights upon great service to the realm, or as an incentive to attract a knight errant of unquestioned prowess to the king's private circle.
In addition, full plate armor is the most technologically advanced armor available in the later medieval and high chivalry settings. The special touches and custom enhancements added by the few living master armorers are what give full plate armor its increased armor class rating over the more traditional forms of field plate. At prices that start at 4,000 gold pieces for a simple, unadorned suit, full plate armor represents the crowning achievement of the armorer's ultimate goal--to forge for man a new skin of steel, as flexible as his own, but as invulnerable as anything in the land.
Field Plate Armor (AC 2)
Description: Field plate is actually a more commonly used form of full plate armor (described later). It consists of shaped and fitted metal plates riveted and interlocked to cover the entire body. Like plate mail, a set of field plate usually includes gauntlets, boots, and a visored helmet (see Helms ). A thick layer of padding must be worn under the armor.
Campaign Use: This armor is rarely used, except by noble knights on a military campaign. In theory, the bulk of a set of field plate armor is so evenly distributed over the whole body that the encumbrance rating of field plate compares quite favorably with that of plate mail and banded mail. In practice, the increased protection is paid for with reduced mobility and increased fatigue.
Each suit of this extremely rare and expensive armor is custom-made and fitted for its prospective wearer. Only a master armorer can create field or full plate armor, and only a master armorer can re-size captured pieces of a suit for a new owner. The new owner must be of at least a similar size and build as the previous owner, or the effort required to modify the piece in question exceeds the expense and effort necessary in forging an entire new set.
Well since this discussion is about plate armour (and AC) and the Dex fighter typically has a lower AC and doesn't wear plate anyway I'm not sure what your point is. I wouldn't say that the strength based fighter isn't worth playing regardless of feats

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.