I think Mastodon's community isn't really up to par with what most Twitter ditchers were expecting.
The Reddit-Lemmy exodus however, is far more exaggerated because of the tremendous number of users on third-party apps that were being killed.
This probably led to a lot more content generation and activity which makes it a lot more welcoming than Mastodon was.
My biggest gripe is that there isn't a working Lemmy iOS app in my region, yet.
I have spotty metered mobile internet connection, and while the web app is lightweight enough, it requires the downloading of page structures each time and it adds up after a while.
I do hope we'll eventually get high-quality apps to interface with the fediverse, much like Apollo with Reddit.