

Because its effort. We have to get the average person to care about their security and privacy before they will bother using these alternatives. It’s much easier for them to download a popular one off an app store and have the data stick with them, than it is to download f-droid, find the right app, make sure its still supported and setup their own data backup.
People are not researching privacy conscious apps and typing it in. Drip isn’t even remotely close to being among the top results for a period tracker. That’s the point, the average person prefers convenience over privacy these days.