why pieces can't be dropped back in chu shogi?

Because Chu Shogi is the game without drops.
It is good because of Lions, not because of drops

There are already too many pieces in chu shogi, and drops on weak spots in the opponent's camp would be devastating.

No, that's not the true reason.
Drops are bad in Chu Shogi not because you can drop them on weak spots but exactly because pieces in game are already too many and if they all remain in game, defending side will easily block any attack and the game would last too long and will be too boring.

I think originally (pre-1500), all the different shogi variants were played without drops. The normal 9x9 shogi we know today was known as "sho shogi" (small shogi) and was considered a child's game, more or less.
Once they added the drop rule to "sho shogi", it became wildly more popular, as the game became quite difficult and interesting. The fast paced nature and sharpness no doubt made it more popular than the larger, more positional, variants.
Plus like @AlexTrick said, IF drops were added into Chu Shogi, the board is simply so large with so many pieces that no doubt each game would be extremely drawish and would just drag out for hundreds of moves...

This topic has been archived and can no longer be replied to.