Because the App is not open source - F-Droid requires OSS software. And yes, there is private repositories that users could manually add, but users won't find you by browsing the F-Droid appstore. F-Droid alone is a tiny fraction of users of Android, and F-Droid users that deal with private repos is a fraction of that fraction. Not worth to bother with that, no one is going to use your app then.