... it wasn't until the 1920s that Western parents began dressing their children in colors. Prior to this, children of both sexes generally wore white, and both boys and girls of a young age were outfitted in dresses. When the color assignments among boys and girls did evolve in the '20s, the colors were reversed: pink was for boys and blue for girls. It wasn't until around the 1940s that the colors flip-flopped to the assignment we recognize today ... (HowStuffWorks)
Whatever the reason, modern marketing doesn't help. I just hope our little one can cope with blue and trucks and dinosaurs. Because she is wearing hand-me-downs from her brothers regardless of the cultural or biological reasons for colour preferences!