As there are around an order of magnitude fewer bluish-violet cone cells than the other two types – and as the other two types are both sensitive to greens – this explains why the human eye is particularly sensitive to variations in the green portion of the spectrum. (For the more pedantic amongst us, the actual ratio of bluish-violet to bluish-green to yellowish-green [i.e., "blue" to "green" to "red"] cone cells is about 1:10:20.)So the reason red and yellow are the top two colors while violet is an also-ran is simple: there are 20 times as many red/yellow-sensitive cones as violet-sensitive ones. By assuming that "red" cones are specially attuned to red and that the three types of cones are present in equal numbers, I made a mystery out of something that's really quite straightforward. Remember what the no-nose guy says, kids!
Now I'm going to go read the rest of that page I linked to. It looks like a very clear and thorough explanation of color.