This is likely due to the response of the cones in the eye. Basically we are more sensitive to light in that part of the spectrum as the different cones* overlap strongly there. Whereas most light in the blue end of the spectrum, for example, is primarily picked up by only one.

It makes sense to spend the additional bit where it will do the most good - SO

see also

• EasyRGB - Color math and programming code examples
• Color similarity/distance in RGBA color space