Many reasons add up. PieFed is a bit better and evolving fast. In my opinion, it is primarily the scale and scope of niche content, and the toxic opinions of many towards genuine people.

A few of the big strikes have been the lack of moderator transparency, and occasional poor moderation. Tribalistic bandwagon behavior, unmitigated negativity by a few individuals, and astroturfing. The inability to block another user from actually seeing and interacting with posts. This place is not privacy oriented. The feed is unmanaged and the effort needed to block and filter the fluff is beyond the curation expectations of many.