Part of it is that women are less likely to join studies (especially risky ones that might impact their fertility or the health of their future children).

Part of it is that men are seen as disposable and it's more socially acceptable to exploit and experiment on men. It was also much easier to deal with men historically since once women got involved everything got a lot more complicated. This was especially true in the past where women were so infantilized that their husbands/fathers were put in charge of their medical care/choices. Those backwards attitudes had some strange consequences. On one hand women were seen as the property of men who could get their wives/daughters institutionalized or even lobotomized for not conforming, but at the same time women were also seen as delicate over-emotional creatures who had to be protected and whose modesty had to be preserved in ways that just weren't a consideration when men were involved. Basically for a large part of our history both men and women have been treated like crap by society and while things have improved in a lot of ways, our records and knowledge have been tainted by those old stupid biases and so we're stuck dealing with the fallout.