I feel there are a lot of philosophical and practical similarities between the FediVerse and the open source operating system world. That's why a lot of people tend to be on both.
The thing about the FediVerse is that you are the only responsible for your feed. Try looking for different content, different people, or different instances, to find more content that resonates more with you and follow those accounts and communities.
I believe it's how the data is structured.
Lemmy is focused on themes and topics, with the "user" not being the focus (you can't even follow a user on Lemmy).
That's reversed on Mastodon, with focus on the users you follow, and the topics (hashtags, groups, etc) being optional.
For some people, Lemmy is better, for others, Mastodon or other microblog platform. The fact that both can exist in the same network is magical to me.