Perhaps because re-apportionment has not favored the GOP anywhere near as much as people had thought. The Dems did a good job in the states they control in squeezing out the most beneficial maps and the Republicans have been stymied in several states where it was thought we would really gain a lot of seats.
Cook and Rothernberg aren't perfect, but they are two of the best in the business. I think Romney could compete and perhaps win, but he isn't a conservative and there is a good reason the GOP base is avoiding him. As we are trying to nominate an actual conservative to represent what is supposed to be a conservative party, I think it is fair to say our chances are sliding due to the relatively poor options we actually had this cycle. I believe Newt gives us our best shot - at least for a hail mary win. Other than that though, there is only Rick left and a Santorum nomination will lead to a landslide GOP defeat of epic proportions. So yeah, I'd say our chances began fading the minute the actual GOP candidate lineup with known - assuming we wanted to nominate an actual conservative anyway.