The wavelength for green (lasers and anything else) is the most receptive wavelength to the human eye (and apparently for raccoons too). What this effectively means is that a green laser will appear alot brighter than a red laser of the same power. Or, a green laser doesn't have to be nearly as powerful as a red one to be an effective pointer. The main reason that red has been used for so long is that red lasers are alot easier to make and are alot cheaper than the green ones.

Greg