Nobody mentioned stripping. A 1-70 twist barrel twist the ball one time in 70" of barrel. A 1-20 twist only needs 20" of barrel for one turn. Most pistols are only half of that. Let's say 10" of barrel. It only spins the ball 1/2 turn with a 1/20 twist. A 35" barrel will spin the ball 1/2 turn with a 1-70 twist.

So, they're both spinning the ball the same amount. The ball needs to spin to be stable. A 1/70 twist in a 10" barrel would not spin the ball enough to be stable.