Tanks are really, really awful. The only reason tanks have an overall increase in winrate this preseason is because no one is playing them.
Because people are running full-damage comps without peel, the teams which have peel in them will win more.
It would be the same if for whatever reason no one played ADCs. Teams that decided to play them and thus have better objective control would beat teams without and ADC winrates would all get boosted, even on the shitiest of marksmen.
People are already shifting focus and ensuring at least one tank to peel per game, and it's showing by the winrate of all but the most broken tanks plummeting.
Realistically, tanks SHOULD have a higher winrate than their counterparts, or at least they should be expected to, because the role of a damage-soaker in some form is vital for a well-rounded team composition.
The true statement here is that there is too much damage early. Even pure tanks have nothing viable for the early game, since the extra armor or MR from the same slot as Conditioning are worth much, much less early on, when health is the more important defensive stat. Tanks lose lane to damn near everything else because they get bullied out from the start. They just win the overall games more because they have a better contribution to their team. It's a situation of losing the battle winning the war.
As soon as tank junglers and/or supports become the common trend, tanks top will lose out even more on win%, since they'll end up behind in their lane, and only contribute as much as the enemy support or jungler in team fights later, while the enemy team will have a cs-filled
or literally any non-tank toplaner contributing more than a tank can.
Just look at some generic picks in the roles and how they relate to fight contribution
vs
vs
vs
vs
vs 
Both teams have one tank, but look at the difference between Riven bullying Poppy and getting the yordle behind, vs two junglers staying relatively close in level, while the rest of the teams are also similar, or hell, imagine that poppy's botlane is up a kill or two.
Now fastforward the game into a teamfight with a level 14
, level 13
, level 12
, and level 13
and think of how that level difference between the tanks and the damage-dealing comboers will relate to a fight.
It doesn't look good for tanks top as soon as the enemy team has a tank in literally any other role, because tanks are complete dog shit early with the runes while damage champs get more than 100 gold extra worth of their offensive stat over Resolve's granted health. So tanks will lose lane, and unless the enemy team has no tank of their own (which is a much more common occurrence in preseason), contribute less overall.