Because a PC monitor does not show a static image - it is being re-drawn every 1/60th or 1/75th of a second (whatever the frequency of the monitor display is).
When you look at one in real-life, the image looks stable because your eye retains an image for 1/24th of a second, so sees a complete image (the brain making sure it synchronises to the complete image, rather than a half-drawn one).
A TV camera records images every 1/25th or 1/50th of a second (depending on the camera type), but does not synchronise to a complete image like your eyes do. So it often grabs an image partly drawn; then the next 1/50th of a second, another slightly different partly drawn image. The difference in the frequencies between the camera capture and the monitor display determine how quickly the 'bands' move down the monitor.
You see the same effect when the TV shows a TV screen. You can use very high-frequency monitors to compensate for the effect.
