https://join-lemmy.org/instances says there's only 2.6k monthly active users in the entire "lemmyverse". That's not even getting off the starting blocks for even a single niche sub-Reddit.
Getting people to even humour the idea of going elsewhere when there's genuine value in pre-existing content is difficult, especially when it comes to troubleshooting, guides, advice et al, which unfortunately, Reddit is brimming with.
Windows Phone was a pretty good example of the "no users" problem: a solid alternative to iOS and Android, but not much in the way of big apps because of a lack of users, but the lack of users primarily stemmed from a lack of big apps. Chicken-and-egg scenario ad infinitum.