the problem with CP in an offense, is he is useless for anything besides running the ball, if he is on the field but doesnt get the ball he is a liability as he doesnt scare anybody as a receiver. so teams have shown that he is easily schemed for as is indicated by his pathetic offensive numbers the last few years.
and yes he is a huge return threat, but he is also more likely to fail to get the yardage that a touch back would have garnered. we have all seen what poor starting field position does to offenses play calling and production, IMO having that 25 for sure beats 1 or 2 TDs a year while failing to get that 25 2 or 3 times per game.