The idea is to have a color which is unique to the background or a portion of the image which is to be removed or altered. Blue is another color commonly used but red is avoided because humans tend to be somewhat pink in our fleshy bits which could be confused for the screen.

Another benefit of using green is that many digital image sensors have more green subpixels than blue or red. This is because the human eye is most able to detect differences in shades of green so maximizing fidelity in that range yields a better image. But this also means that cameras effectively have a higher resolution for the color green so a mask made using that color can be better than with a different color.