Because heavy tank destroyers and assault guns are only in 5 out of 71 commanders. Heavy tanks are significantly better represented at 9 (15 if we count the KV-2, Crocodile and AVRE) commanders.
Furthermore, heavy TDs are problematic in 1v1s because of limited use/performance and not because of timing issues, while heavy tanks currently perform well across all gamemodes and (arguably) only timing is an issue. It's already been proven long ago that the heavy TDs would never work across all gamemodes, because they aren't versatile enough (without being OP), while there is still a lot of wiggle room left for heavies.
I see no reason to give up already and condemn another 9-15 commanders to go out of meta in some gamemodes (the most popular ones at that) when there's still a good chance that some minor adjustments will make them fit in more easily.
Bet way to adjust between game modes is cost and CPs. 300 fuel in 3v3/4v4 is nowhere near as crippling as in 1v1 due to inflated income. Therefore you can make heavy tanks and heavy TDs into good units, then adjust their 1v1 importance via cost.