The ability for tanks to layer abilities in a way that removed nearly any and all built in weaknesses of each other, ultimately created a scenario where large chunks of the playable cast simply did not function in given meta.
As it inherently negated any effect of heroes purposely built to play into those weaknesses. For example in ye old OW1, Mei could force a d.va out of space in threat of a freeze. However in the Dive duo of winston/d.va the d.va would almost always be engaging under a winston bubble, which would break LOS of the beam simply moving in and out of dome.
This also removed any need for d.va to worry about cross fire from multiple directions. And Winston needing to worry about the likes reaper/hog/torb from entering the bubble, as the d.va would consistently be able to peel off the threat without issue via DM and the two layering damage with the rest of their team.
This sort of interaction between tanks is inherently what nearly every meta was built around. Usually forcing the investment of multiple ults just to break through a given sets advantages, Limiting most play in between to mostly just poke to gain said ults. As for most scenarios it was the only reliable way to break through a given comp.
Were the list of viable heroes simply broke down to who can sustain the meta comp, and who can set up the most reliable ult combo to get through such.
And this simply isn’t something balance patches could fix, as the issue existed in how theses characters interacted with each other. And not in say how strong a given ability was at the time.
That and any attempt to nerf the meta tank line, would in turn severally weaken those tanks in any context that was their meta parring. And in turn created a new issue in that players being parried with people that didn’t want play their matching hero or were not skilled enough on said hero to be effective. Doubly so when you take account that you also need to be aware in how those tanks actually play with each other, as much as how each of them function.
Creating many one sided games the moment players were placed into your que, simply because of different tank interests or god forbid them being mains of the same hero.
This in combination with the shrinking tank player base, is why 6v6 simply isn’t a viable mode. As the more heroes they add the smaller the design space for tanks and in some cases supports gets. Their just every more increasing interactions they simply can’t have in game without enabling another tank DUO that would cover each other’s weaknesses.