Why aren't there more knuckleball pitchers in the MLB (since its such an effective pitch)?

The knuckleball is a certain pitch where there is very little spin to it, but it is an effective pitch because it is hard to predict where it will land. Usually, the batters will take a swing at a knuckleball only for it to change its path and is missed by the batter causing him to get a strike. Many pitchers try to perfect their knuckleball pitches. Even though it seems that pitchers would always use a knuckleball, they don’t use it as often in the MLB for several reasons. Pitchers have trouble controlling the ball when they are about to throw a knuckleball pitch.

Also, catchers have trouble catching a knuckleball pitch, so if the batter tries to hit a knuckleball on the 3rd strike and misses, the catcher could miss catching the ball and the batter might be able to run to first base.