I know the fact that a slow left arm bowler is always easier for a left handed batsman. I have seen several matches where captains won't give chances for slow left arm bowlers to bowl against left handed batsmen. During India - Sri Lanka matches, captain Dhoni didn't give much chance for R Jadeja. Why is it easier? Does it also apply to right arm spinners to right handed batsmen?
The answer lies in the fact that it is considered to be harder to play a ball that is spinning away from the body than a ball that is spinning in towards the body.
Therefore off spin and chinaman bowlers, who naturally move the ball away from the left-hander, will be expected to be most effective against left-handed batsmen. By contrast, leg spin and slow left arm bowlers, who naturally move the ball away from the right-hander, will be expected to excel against right-handed batsmen.
The natural tendency of a batsman is to hit the ball on the leg side. This is because it is easier.
It takes much more effort(on the wrists & arms) & skill(feet movement & body position) to play the ball on off side.
So, leg side hitting easier, off side hitting relatively difficult.
Then comes the question of hitting with the spin & hitting against the spin.
Hitting with the spin is easier because you already have the momentum of the ball going in the direction you are hitting & you can use the speed of the ball.
Hitting against the spin is difficult because you have to change the momentum of the ball in opposite direction & generate your bat-speed to drive the ball past boundary.
So hitting against spin difficult. Hitting with the spin easier.
Hence slow left arm spinners are generally not used against left hand batsman because it would be easier to hit with the spin & on the leg side.
Both are easier as compared to hitting against the spin(ie square cut/cover drive) on the off side.
The ulterior motive is also to get the batsman out through LBW which is easier for right arm off spinner against left handed batsman. But that depends on the skill & deception of the bowler