Reddit/Lemmy are link aggregators with a forum-like comment and post structure. Mastodon/Twitter are microblogging platforms. You go to someone's blog to read their thoughts, in the context of themselves, and everyone choosing to follow would like to keep up to date with their going ons/niche interests. You go to a forum to read discussions, on a specific topic, in a (somewhat) more organized fashion, and will recognize the regulars in regards to their discussion contribution.

On Mastodon/Twitter, you follow people. On Reddit/Lemmy, you follow topics. You can follow hashtags on mastodon, and you can follow people on reddit, but in general, the philosophy of what you would do with these platforms is different. These both can work together on the fediverse, and in general, social media, because a post on either isn't much more than text and images with some categorical tags/filters. The technical specifications aren't that much different, so they can be applied in the same space.

However, I feel like lemmy and mastodon aren't going to see as much interaction with each other for this reason. It's possible, people have already been demonstrating it, but I've tried browsing a community from mastodon and it's just not inductive to how that UI/strategy displays long form comment chains. I mean mastodon itself doesn't even create a visual indicator of comment chains. This isn't exactly a terrible downside, I do like mastodon and used twitter a lot, but I go to lemmy and mastodon for different reasons, as they were created for different purposes!