Then the question is, why is it when users come from reddit they don’t hang around? they don’t tell their friends, hey this is a place that’s awesome!

Some do. I would argue that most social media websites sign-ups become dormant accounts very quickly. Whether its Reddit, a large Discord server, a reddit alternative etc. The retainment rate of most social media sites is and has always been quite bad. People are fickle, or not as interested as they thought, or just sign up to have a better look. I don't think there's anything special about Lemmy or Piefed here, and advertising it isn't exactly easy nor even welcome.

Does Lemmy/Piefed have the best design? No. Is it uniquely bad? Not at all. I think Reddits is pretty poor in areas, but that doesn't seem to be an encumbrance to it.