I asked my dad (an eye scientist) a while back, and here's what he told me. (If I understood his dumbed-down version.)
You know the human eye perceives light through its rods and cones. Rods' peak reception is more or less at 502 nm. Green cones at 534 nm. Red cones at 560-ish. You have more green cones than red.
Background (that I actually am just giving from memory): Rods don't perceive color, just light/dark. Rods are saturated during the daytime and don't contribute much to seeing a wavelength-specific light like a laser. They sure help in the nighttime, tho.
As noted above, the wavelengths of the lasers in question (pulled off various makers' sites) is 532 nm for the green laser wavelength and 650 nm for the red laser.
The reason the green laser is easier to perceive at a given power is that it's closer to the peak of the green receptors than the red laser is to the peak of the red receptors. Further, you have more green receptors and the green laser is also closer to the rods' reception freq.
Dad says, red/green color-blind is sort of a different deal (I asked him not to elaborate), and that the basic explanation he gave to me works for 99.9% of people. Getting back to the rods, he said that you can diminish the power of a green laser about 100x more than a red laser and still have it be visible to the eye as light (although not colored light), just because the rods will pick it up easier since it's closer to their peak reception freq.