Trackpads can't mimic the precision (a dot the size of a pinhead vs a fat thumb). They can't get rid of their terrible z-axis problem (far worse than mice -- and not present in trackballs). They also can't handle inertia well due to lack of feedback (When you use a trackball, you become very adept at flinging the ball and stopping it at the exact position you want).
As to pro-gamers, some of us remember when over half of the pro gamers used nothing but trackballs (in preference to optical or mechanical mice). The short story here is that trackballs didn't keep up with the newer laser technology needed for better resolution, so the pros moved on to mice. There are now some trackballs that use laser technology, but everyone (for better or worse) has moved on.
As to pro-gamers, some of us remember when over half of the pro gamers used nothing but trackballs (in preference to optical or mechanical mice). The short story here is that trackballs didn't keep up with the newer laser technology needed for better resolution, so the pros moved on to mice. There are now some trackballs that use laser technology, but everyone (for better or worse) has moved on.