This is not the .20 thread this is why does .177 exist
But to answer you,
What .20 has done is prove itself as the best compromise FOR UK HUNTING AT SUB 12, because it's more efficient with air, retains more energy %, hits hard, flies flat & is the best balance across the calibres when the muzzle energy is fixed.
The reason it's the best compromise is because every other calibre will come last in one comparison test, but .20 doesn't.