I think it’s not so much the shampoo but a mix of most men using shorter haircuts and doing less damaging things to their hair, like hair colouring.
And it’s not like there aren’t specialized shampoos targeting primarily men. I think I never saw a dandruff shampoo marketed towards women.
From what I could find, men are more likely to suffer from dandruff than women, so even though women can suffer from dandruff it makes sense from a marketing perspective to focus mostly on men, especially since men are still thought that anything even cosmetics adjacent is not manly.