The reason frozen heart is underperforming.

This is an item that gives cdr. mana, armor, and a passive that slows attack speed in an aoe.

you would expect this to be an item you build if you are against multiple ad auto attackers, yet this item is really unpopular compared to how it used to be.

Lets do some math, Lets just say that the tank has about 2000 hp and 60 armor at a given time. The new frozen heart buff + the base armor will knock the effective hp up to 170% of what the hp is, so 5400

Keep in mind that hp amounts and armor amounts vary from champion to champion so because effective hp is calculated based off of ratios, it is possible that certain champions may end up with numbers of effective health greater or lower than other champions.

lets look at other armor items now.

Sunfire cape gives 60 armor and 425 hp. so for the calculation we start with 2425 hp and increase it by 120%. You end up with 5335 effective health. Significantly less, but you end up with a unique passive that allows you to kill minions faster that doesn't force you to put abilities on a cooldown.

Randuins gives 400 hp and 70 armor. so 2400 armor increased by 130 % is 5520 effective hp. Alternatively, you gain the attack speed slow if you are hit, AND crit reduction making it a better choice against high crit teams.

thornmail start with 2250 hp and increase by 60+80 = 140% == 5400 effective hp. So its basically equivalent to the frozen heart effective hp, but there is a really important reason why you would never want to go frozen over thornmail.

Any champion that frozenheart is theoretically good against, thornmail is a better buy. When we think of toplane auto attackers, the overwhelming majority of them run conq or have some other form of self heal. In that case thornmail is 100% the better option. It offers both the attack speed slow and grievous wounds, making it a significantly better option when playing into auto attackers. There is also a lot more true damage in the game than there used to, so an hp item that brings you ~250 hp in front of frozen heart ends up being an ignites worth of difference in effective health. The only time you would really consider getting frozen heart is if you are a tank who is playing against a full ad comp. If the enemy has an ap champion it ends up being more effective to buy an mr item so you dont get one shot by ap damage, and the hp gained from the item will factor into the effective health ratio for armor as well.

I mean its kind of cheap and the mana and cdr are good for waveclear, but at that point you might as well just go sunfire cape because it offers better waveclear than any tank ability. Not to mention that you have to put your abilities on cd if you want to waveclear with frozen which can result in death in a lot of lanes.

IDK, too much self heal in the game. If we ever go back to a tank meta where games and fights last longer, and there are no more instagib cheesers this might be an item that comes back given that the attack speed aoe slow tends to be good against people who aren't attacking you, but for now, too much true damage to allow pure armor items through, and too much self heal to not build thornmail.