Not sure about you guys, but as I've watched games it seems like the better hitters get (i.e. more batspeed they have) the more effective the changeup becomes. Probably because a slow swing stays in the hitting zone a long time, so there's more margin for error. If the hitter is just a little behind the fastball/speed pitches, the changeup sometimes slows it down just enough to be hittable.
As they get better, hitters pick up bat speed. Which means the bat is not only into the hitting zone quicker but out of it quicker too. If the pitcher does a good job of disguising it the hitter swings ahead of it. Another thing that can happen is the hitter freezes. She recognizes something's not right but by the time she processes it the ball has gone by. A less-sophisticated hitter may not be capable of recognizing the difference so quickly and is just swinging when she swings, hoping something good will happen - even if by accident.
As they get better, hitters pick up bat speed. Which means the bat is not only into the hitting zone quicker but out of it quicker too. If the pitcher does a good job of disguising it the hitter swings ahead of it. Another thing that can happen is the hitter freezes. She recognizes something's not right but by the time she processes it the ball has gone by. A less-sophisticated hitter may not be capable of recognizing the difference so quickly and is just swinging when she swings, hoping something good will happen - even if by accident.