There’s a claim that it was a marketing scheme in the 1940s to reduce the usefulness of hand-me-downs in families. My grandmother would have lived through that and I may see if she remembers anything about it. She was definitely babysitting or watching children by 1940.

That doesn't make sense to me US textiles were in high demand, though perhaps not in pink and blue starting around 1940 and by the end of the decade US consumers were getting quite wealthy. I'm not saying it didn't happen but I'd like to see a better reason than "companies love money" since if you loved money in the 1940s there were better ways to get it than trying to do some sort of marketing campaign to reverse a social standard (using a marketing industry that was much less advanced and pervasive no less.)