Challenge during Panthers game

Since when is it a rule that the defense cannot challenge whether a pass is incomplete or not? Why not? If the defense contends the ball was a catch and a fumble (which they recover), why can they not challenge the official's call of an incomplete pass? The challenge rules are so convoluted that it leads to nothing but judgmental control of the game by the officials based on when they decide they want to blow a whistle or prevent a challenge.