I think it's a matter of barrel length and potential velocity. Both the 1:20 twist and the 1:70 twist will spin the ball so long as the patch/ball combination grips the rifling.

A 1:20 twist works in a short pistol barrel with a light powder charge because the velocity is low enough for the rifling to maintain its grip on ball/patch. However, the powder charge normally used in a rifle will cause the ball/patch to strip the rifling in a 1:20 twist barrel.