I think you are missing the point here. The health app itself won't get interest from 100% of their iPhone users either. I'm assuming they simply didn't want to introduce a gender specific function in their app that would fragment more the users of that app. If you take the number of iPhone owners, who would have an interest in the Health app and then you cut again to get only the women who would be interested in tracking their periods inside their phone, I think you look at a fraction of their users. Not that those users would be any less valuable or anything but they probably didn't represent a demographic big enough to justify a dedicated feature. Especially since they would probably still use the app anyway for all the other features it proposes.
fataliss|11 years ago