It has been mentioned here before that the role of the big is evolving. They are expected to be far more mobile and versatile than they traditionally would have been. Much of the play now involves the bigs setting screens on the elbow to open up the play for dribble penetration. Nothing wrong with it but it effectively boxes them out reducing rebound opportunities.
Along with the ever increasing number of three point shots being taken which result in big rebounds falling less predictably and well away from the ring. Not saying it is wrong, but judging a big on their rebound count is a bit unfair when many coaches run plays that are not designed to maximize their opportunities. you have to get your hands on the ball to rack up stats, stats only tell half the story of the impact on the game. In saying that the other half can be shit too, better judge is if the team looks better with them on or off the court.