Boiling everything down to mostly two color schemes means less worries about what will sell. You can have larger production runs and yet end up with less overall stock. And once you have convinced everyone that boy == blue and girl == pink, it means there is a lower likelihood that the clothes will be reusable by the next offspring.

Going out on a limb: once something like this has reached critical mass, it is hard to go back, or take it in another direction. The colors have become intertwined with gender identity and that is a rather touchy topic to this day.

(There is some research in the matter - see http://www.smithsonianmag.com/arts-culture/when-did-girls-start-wearing-pink-1370097/, but that cover blurb also does not reveal how it was cemented in place in the '40s.)