If you read the wiki article department stores wanted to collaborate on colors. Most stores were doing all colors for babies, but it’s easier to mass produce one color per gender. There was a time where different stores were doing boys pink (because red is masculine, so pink is baby red) and girls in a feminine light blue. Eventually they settled on girl pink and boy blue. It was utterly arbitrary. Yellow was used for gender neutral.

It’s only recently with online shopping that we are getting back to being able to buy baby clothes in any color once again.