Probably because of the space between the sensor and the glass of the camera. Pics of the unassembled camera here:
http://www.cctvforum.com/viewtopic.php?f=19&t=37006
You could try to change the lens on a 2CD20xx youself for a 2.8mm lens. Might it work? It sure might work. But as I said, you might be unable to focus the lens (it will hit the IR cut filter before it is in focus) or you might get a tunnel effect. I would be surprised that noone has tried it before.
On that thread you will see someone did change the 4mm lens for 6mm, 8mm and 12mm.
Quotes:
"The 1 / 2.5” format ensures that the lens introduces no dark corners in the image, and that the distortion is also lower because the lenses are using the most central part."
"The dimensions are very similar to the original lens of HIKVISION which will help to not collide with the front glass of the camera."