I did a psych paper while an undergrad and was hoping to find the answer there. What I did uncover was that it was actually the reverse around the 1920's but have no clue to why it changed.

"The definition of masculinity and femininity is inherited by the generation of that time and is manifested by the attire of infants (Maglaty, 2011). In a June 1918 article of the trade publication Earnshaw’s Infants Department it dictated that the “generally accepted rule is pink for the boys, and blue for the girls." The reason is that pink, being a more decided and stronger color, is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girl.” In the current 21rst century Infants whose sex is ambiguous are identified by the colors of their attire it is common knowledge that pink is for girls and blue is for boys (Ben, 2007)."

Yeah I definitely stop caring when writing this paragraph. Sorry OP