Getting your app published on Google Play is fairly easy. You basically just upload an APK and with how little Google reviews apps, you can get almost anything published. F-Droid on the other hand has very strict standards (which is a good thing). Since I published an app myself on F-Droid, I can tell you that fulfilling those standards does take a lot of effort, and thus a lot of love from the developer.

This creates a filter: If you care enough about your project to go the extra mile to fulfil the F-Droid requirements, you probably also care about making a good product. If you don't, you also won't be motivated enough to publish on F-Droid. There are countless open-source projects which sort-of work but are left behind, usually because it was someone's side-project. This is ok as well, because it's free time that most people are spending on Open-Source projects, but as a result, most open-source projects are actually not very mature. They just never make it onto F-Droid and don't go viral.