To sum it up
Crappy Matchmaking
Early game focus
Snowball heavy
Extreme amounts of damage
What this means
Crappy Matchmaking:
You get matched with clowns, tanks are reliant on team more so then any other class, a bunch of feeding clowns is the worst thign which can happen to you when playing tank it leaves you powerless to affect the outcome of a game.
Or you get matched vs clowns but you can't take advantage because you are a tank
Early game focus:
Riot made this game all about the early game, this ties in with the snowballing aspect too, Most tanks have a shitty early game vs bruiser or carries who have a good early game and are better suited for trading, in the jungle their clears are weak which leaves them vulnerable to invades and their laners vulnerable to ganks without you as playing tank capable of doing anything about it. So in short, you start the game at a heavy disadvantage yay!
Snowball heavy:
Because riot made this game so very snowbally, having a great early game is important, which i pointed out in my previous point. but what is worse, tanks snowball poorly, so even if you can get past the shitty early game successfully it still wont have a major impact vs say an ADC or Mage snowballing. You wont get much better at taking objectives or farm any quicker either compared to those classes to further amplify your snowballing. SO even if you do everything right you can't carry thus are still reliant on your team, which ties to point one, if they are clowns youre still fucked, if the enemy is clowns then you being fed still wont matter much anyway.
Extreme amounts of damage:
Since the end of season 5 tank items have been nerfed as have tank masteries, you now scale more poorly then ever. In contrast damage skyrocketed, with ADC as a class being reverse sejuani'd by removing ad from items then giving them more base ad, then buffing the items again, as well as gaining a ton of crit, and a crit runaans(fucking bullshit whoever thought that was a great idea should kys) and gaining more damage from masteries then before, leaving some adc end builds with quadruple the teamfight damage they used to bring, even mage damage went up due to masteries.
So tanks never really get to fullfill any sort of endgame fantasy anymore, they will never reach raidboss status, in fact they drop off extremely hard during lategame, leaving them only with some power in the midgame for which they first must overcome their weak early game. And even then they are still team reliant so even at their only time to shine, midgame, they are mediocre at best.