Tanks were actually in an alright spot in season 5 after cinderhulk came to power. Yes, it broke a few champs (as is normal for anything Riot does these days) but they got nerfs specifically to deal with their interaction and were quelled in a few patches.
For the rest, cinderhulk allowed them survive longer than five seconds in a teamfight. Cinderhulk allowed tanks to contain assassins instead of being food for them. Cinderhulk allowed tanks to achieve "tanky" status quicker, and thus made being a tank more fulfilling. It made being a tank mean something; specifically not that you were an extra two or three autos/spells from death compared to a squishy mage like in season 6.
Being in a real tank meta (as opposed to a meta where everyone is tanky, as there's a huge, important difference) means that tanks being popular made the game more diverse. Assassins were still viable, but not as popular, so champs vulnerable to assassins actually saw play. Support tanks were starting to lose their place as king because they couldn't buy cinderhulk, and we started seeing more caster and healing supports in addition to tanky supports. With teams having at least one, and often two tanks, carry tops and junglers were starting to appear with about the same frequency as tanky ones.
In fact, cinderhulk was such a success that top laners were taking smite just to buy it and feel like a tank should. Did Riot do the right thing and make cinderhulk into an item that solo laners could buy so they didn't need to waste a summoner spell? Hell no! They were offended that laners were buying a jungle item and nerfed the crap out of it (without buffing the champs that got nerfed for it back up, hello Sejuani).
And then with the nerfs to cinderhulk and the newly released sated devourer, tanks were pretty firmly kicked out of the jungle and the season 5 meta went from "Hey, lots of different things are viable now" to "Get sated, win game" and then stayed there until season 6.
Then in season 6, marksmen got all kinds of buffs to their kits, mechanics, and itemization and they all started ripping tanks in half with more ease than ever before. To combat this, Riot stupidly decided not to make tanks tankier, but to give them more damage so they are more of a threat. That's great if you're Mundo, but not every tank is Mundo. In fact, Mundo is more of a juggernaut these days; which is what Riot attempted to turn a lot of tanks into with juggernaut items to band-aid solve the problem of them being next to useless.
Tanks didn't need more damage, they needed to survive and be a slow, grinding threat. Cinderhulk encouraged that when it was strong, and that's why tanks actually accomplished things with it.