Several reasons I think one can logically assume is going on here.
1. Jerry and NBC want the blimp shots of the stadium and that only happens with the roof open.
2. We are trying to make it seem like an "outdoor" game since the Saints are not as good outdoors as compared to playing in a dome.
3. Crowd noise, unfortunately from the Saints fans. With the roof closed, the noise will be reverberated back down and will increase the noise level on the field. If 40-50% of the fans are Saint fans, that can affect the Cowboys on the field.
4. Jerry started running the numbers on how much it takes to run the AC for 3 hours and said screw it, LOL