The second a corner turns his head to track a ball, he automatically slows down and the result is the WR instantly gains a step of separation on him.
Modern NFL offenses are so tightly co-ordinated and QBs are now so accurate, that this single step of separation is now a significant advantage. A corner turning his head back is far more likely to lead to a big completion than to a pass break-up.
Corners are now coached to keep their head forward so they can maintain top speed and track WRs, then when the ball comes in to get their hand(s) into a WRs arms to rip and tear and prevent catches that way.