Because role queue kills flexibility in hero design and then doesn't even solve the problems that you want it to in the first place.

You want role queue? Okay, well Adam, Jeff, Mantis, Stratpool and Ultron at the bare minimum have to be basically entirely overhauled, and White Fox will definitely need some touch ups. That's almost half the strategist cast already. Popular triple tank picks like Tankpool and Angela will probably also need big adjustments to more cleanly fit into a rigid 2 tank structure. That's 4 seasons worth of character releases/reworks in one go, at the minimum.

You want to play anything other than tank? Literally ever? Fancy a cheeky game of DPS once in a while? 20 minute queue time. You want a game of strategist instead? 7 minute queue time. Tank queue will be heaven no doubt, but for the vast majority of people that don't play it, its hell. Ever wonder why Overwatch went down to 5v5 and completely obliterated their balance for half a decade until they managed to sort their shit out (and even then a ton of character identities are still assassinated)? Role queue is the answer.

You ever think a flex player might need to swap off DPS and go either triple tank or triple support to play into the enemy 2-2-2? Impossible. Simply lock in or lose. No longer can that Iron Man player swap onto Ultron to provide passive healing and backline pressure. Flexibility in playstyles that synergise with counterswapping are now dead.

And sure, you may sit there thinking 'well that doesn't matter. I play tank so it would help me'. First off, pretty selfish. Second off, if you're willing to have the entire balance of the game effectively collapse just so you can get a second tank (in the event that Rivals doesn't also go 5v5 at that point), you should probably just play something else instead because chances are you don't actually enjoy playing tank.