Lemmy is a much closer analog to Reddit than Mastodon is for Twitter. While Mastodon has similar basic functionality to Twitter, it lacks a lot of the features that make it easy to find new content and new people to follow.
Pair that with some very polished third-party mobile reddit apps with large, loyal followings transitioning to Lemmy and it became way easier to abandon reddit for Lemmy than it was to leave Twitter for Mastodon. I’m a huge open source supporter, but the average user doesn’t care about FOSS or open source software. They want something that looks nice and just works.