Probably because it's kind of a bad experience? I say that as someone that uses it regularly for multiple apps. I'm not a fan, the search alone is basically useless unless you know the exact title of what you're looking for and even then it's sometimes not the first result for some reason.

I'm sure that someone will reply to this with some suggestion for some alternative front end for the fdroid store that supposedly fixes that which will just further highlight the issues with it of fragmentation