Tanks have been shit since the midseason 7 tank update 7.6 fucked their itemization, and then they got double fucked when new runes removed 32 scaling MR, around 30 early Armor, and 50+ early health between runes and masteries.
Adding armor back into rune shards was scant compensation since the amount of armor is 240 gold worth while AD shards get over 350 extra gold worth of stats at the very start of the game. All of the tank runes that actually provide tankiness are conditional with high cooldown (Shield bash, Aftershock, Bone Plating), or only activate once laning phase is long since over (Overgrowth, Conditioning)
Couple an early game stat deficit with Damage outscaling resistances at a rate of 1.5:1 and final tank items being both less gold efficient and much less slot efficient than offensive equivalents, and you get a game where tanks are not viable for solo queue unless they're building full damage themselves (A million parentheses (
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There's also the more general issue where the stats tanks have to build; health, armor, MR; provide no wave clear or objective taking power, while carries build stats that both help them win trades AND take objectives. And of course, the fact that carries can build a single stat, or two at most, while tanks have to split their buys into both resistances and end up unable to take both types of damage as well as they could. And, of course, "as well as they could" would be 66% as efficient as, say a 5 AD comp from the enemy.