Now that most of you mention it, the absence of the video wall is taking longer than expected. They should have fixed it by now.
I think it's because of several reasons.
First, the video wall, like many other kinds of video displays, has grown dimmer over time. It requires a very bright display to overcome the effects of the bright studio lighting and still be picked up by the camera.
Second, a video display will never appear quite as sharp on camera as directly-generated computer graphics, such as the chroma key wall. Crudely speaking, think of copying one videotape to another. The second generation is going to be somewhat worse in quality than the first. That's the general idea here. Instead of looking at the video wall directly with your eyes, the image from the video wall has to be captured by a camera, converted to an electrical signal, sent to the broadcast equipment, transmitted via satellite and cable, reproduced on your TV screen, and only then seen by your eyes.
Third, I'm sure the budget does come into play here, as refurbishing the video wall can't be cheap.