A relevant f-droid search:
* https://search.f-droid.org/?q=Menstrual
All of the apps that showed up in the search store data locally. Why would anyone not want to store this sort of data locally? What is the advantage of sending the data off to a server somewhere?
I don't have hard data, but I think I'm correct in assuming that the overwhelming majority of F-Droid users don't menstruate
Because your typical android user is not even aware of it and if they are they don't know or care enough to look for an alternative.
People have multiple devices, want to make the data available to their partners, etc. It's a legitimate but fragile use case.