The retention is bad because the new user experience is abysmal. It takes a lot of effort, even for a tech savvy user to get Lemmy & Mastodon to a usable state. Most people are not going to do that and will churn out when there's no interesting content and the app does weird things like shows you the same posts over and over, if you don't manually change settings for things like "hide read posts" in Voyager.