Here is an academic medicine perspective: https://www.aamc.org/news/why-we-know-so-little-about-women-...

To give you some TL;DR from personal-ish experience, women have historically been excluded from medical trials because:

* why include them? people are people, right?
* except when they're pregnant or could be pregnant -- a trial by definition has risks, and so "of course" one would want to exclude anyone who is or could get pregnant (it's the clinical trial version of "she's just going to get married and leave the job anyway")
* and cyclical fluctuations in hormones are annoying.

The first one is wrong (tho is an oversight that many had for years, assuming for instance that heart attacks and autism would present with the same symptoms in all adult humans).

The second is an un-nuanced approach to risk. Pregnant ladies also need medical treatment for things, and it's pretty annoying to be pregnant and be told that you need to decide among unstudied treatments for some non-pregnancy-related problem.

The third is just a difficult fact of life. I know researchers studying elite performance in women athletes, for instance. At an elite level, it would be useful to understand if there are different effects of training (strength, speed, endurance) at different times in the menstrual cycle. To do this, you need to measure hormone levels in the blood to establish on a scientific basis where in the cycle a study participant is. Turns out there is significant heterogeneity in how this process works. So some scientists in the field are arguing that studies should only be conducted on women who are experiencing "normal menstrual cycles" which is defined by them as three continuous months of a cycle between 28-35 days. So to establish that then you've got to get these ladies in for three months before the study can even start, getting these hormone levels measured to establish that the cycle is "normal", before you can even start your intervention. (Ain't no one got $$ for that...) And that's before we bring in the fact that many women performing on an elite level in sport don't have a normal menstrual cycle. But from the sports side, they'd still like to know what training is most effective.... so that's a very current debate in the field. And I haven't even started on hormonal birth control! Birth control provides a base level of hormone circulating in the blood, but if it's from a pill it's varying on a daily basis, while if it's a patch or ring it's on a monthly basis (or longer). There's some question of whether that hormonal load from the birth control is then suppressing natural production of some hormones. And why does this matter? Because estrogen for instance has significant effects on cardiovascular health, being cardioprotective from puberty up to menopause. (Yeah, I didn't even get started on perimenopause or menopause.)

Fine, fine, it's just data analysis & logistics. If you get the ladies (only between 21-35) into the lab for blood samples frequently enough and measure at the same time of day every time to avoid daily effects and find a large enough group that you can dump all the ladies who don't fit some definition of normal & anyone who gets pregnant but still get the power for your study, it's all fine, right? You've just expanded medical research to incorporate, like, 10% more of the population....!