Every calibre is a compromise with the fixed control of sub 12 power
With .177 using a heavy pellet for more "thump" you lose the velocity & therefore lose the flat trajectory
With .22 using an ultra light pellet for better velocity, it doesn't have the mass to retain it's energy so will lose momentum quicker & retains less impact energy.
Each calibre has an optimal range of pellet weight to give it's best for the sub 12 power limit,
and it just so happens that .20 gives the best overall compromise of velocity, weight, momentum & surface area at that power level.
It's simple schoolboy physics, nothing more - nothing less.