For me:

1. Privacy reasons. When a comment is "deleted" on Lemmy, the comment is actually only hidden to all except instance administrators. The comment remains on the post and continues to display the poster's username. kbin is also not a beacon of privacy, but it at least removes deleted comments from threads. This is also why I try to interact more on kbin magazines than Lemmy communities.

2. kbin has a sweet community search tool that not only searches kbin magazines, but also Lemmy communities and even Mastodon groups. This means you can easily find communities all across the #Fediverse for any of your interests.

3. kbin has a much nicer/more modern UI. It's got some quirks, but it's easier to read and navigate than Lemmy by default.

4. Customization options! Lemmy has themes, which is cool, but kbin has themes and lots of fun toggles to change your experience.

5. Last but certainly not least, Lemmy devs have a pretty shit stance on human rights. (See here: https://mstdn.social/@feditips/106835057054633379). There are communities like #Beehaw, which are super friendly and non-problematic instances separate from the Lemmy devs, but it's worth noting that instances like Lemmy's flagship instance and Lemmygrad are run by folks with some grossly misguided views.