Because Lemmy, to this day, doesn't do what Reddit does. Yes, the UI is similar, but there's two big downsides to Reddit. One that's important now, and one that's important later.
- Lemmy is tiny. Like, really small. The Linus Tech Tips form and the Crackberry forum each have more users and more activity than all of Lemmy combined. That means, you can talk about general things of Lemmy, like e.g. US politics, but there aren't a lot of niche communities. On Reddit I can post a photo with some weird electronics component from the 60s and within minutes someone will post an answer identifying the component and telling me where to buy a replacement. On Lemmy, a corresponding community doesn't even exist.
- Lemmy scales terribly. Every instance holds a copy of all data that was ever posted in any community that any user on that instance ever subscribed to. That has two very negative effects:
- Storage requirements are insane. Since most traffic is in big communities and most users will subscribe to the big communities, most instances need to store a copy of almost all of Lemmy. If Lemmy were to ever get to the size of Reddit, every instance would have to store data in the order of magnitude of all of Reddit. Imagine small hobby admins having to host data in the region of Petabytes or Exabytes. Nobody can afford that.
- Admin work is insane. Since every instance holds a full, independent copy and doesn't only cache, they are legally responsible for the content and have to moderate it. So if someone posts e.g. illegal pornography on one instance and it's federated to another instance, the admin of the second instance needs to delete it or face legal consequences. That means, instead of the mods or admins of the original community/instance being solely responsible for keeping their stuff clean, everyone is responsible for everything and the same work needs to be done hundreds of times, once per instance.
This horrible scalability means that right now instances are getting close to their limits (see e.g. lemm.ee closing down exactly due to these reasons).
Lemmy has 40-50k monthly active users. Reddit has 5.16 billion monthly active users, so about 100 000x. If everyone on Reddit were to move over to Lemmy, Lemmy would be done. Just one day of Reddit-level traffic would be enough to jam up the history of Lemmy content so much that nobody could ever afford hosting a Lemmy instance again.