Because he was playing him early. What happened? holding penalty at first, then completion where he just owerpowered and outjumped him, then next play completely busted coverage (if Romo saw Dez, would have been TD), next two plays Dez was deep and had several yards on Peterson, again Romo not seeing that... One of those easy TD again if the ball was thrown.. Romo missed two TDs in just that short amount of time of those two against each other...
Much better to have your next best CB covering and have a guy on top, it was a mismatch to play like that all game long. You only do that if it is effective, it has to make sense matchup wise. I prefer to do the reverse, let my #2CB cover their best wideout and get him much over the top help, and then let the best CB cover their #2WR with less frequent over the top help, I think this is more effective.
besides, we no longer will be putting the pressure on QB as we did with Horton, and it will be harder for CBs to stay in man coverage for long. Last year, the effective blitzing schemes masked the need many times for down the field man coverage without help over the top. This year, we will play with more sover the top safety help, and than instead relying on the 4 man rush to apply pressure. It is a completely different philosophy. Corners will need to cover for more time now, but, with the added help of more safety help. I think this is already obvious at times.