I was wondering, why is it that?

Question

Left-Handed Sweeping Curves work well against lefties but right handed sweeping curves don't work well against righties??

Answer 1

because if you think about it left-handed hitters like the ball down and in, a left handed sweeping curve goes down and away to a left-handed hitter. right-handed hitters however are more accustomed to going the other way while left-handers pull the ball the majority of the time, which is why switch-hitting is a good skill to have, then you never have that problem.

Answer 2

The righty-righty matchup isn't as tough for a hitter because since the majority of batters are right-handed and the majority of pitchers are right-handed, they are more accustomed to hitting the curveball coming from that angle.  Lefties don't hit off left-handed pitchers near as often, so they just aren't used to it.