* Lemmy has much more "extreme" roots. The original people who went to Lemmy back in 2018/2019 have very strong political ideologies, and even though Lemmy is much bigger now, those roots really shaped the culture of Lemmy
* Lemmy is federated, sort of like email. You can self-host your own email provider if you really want, but you should probably just register to one of the top 3 providers
* Lemmy is missing niche communities. At around ~1% the size of Reddit, you just won't find a community for your favorite game, city, or meme genre
* Lemmy is obviously open source (AGPL), and their servers are very cheap to run (compared to Reddit's insane op costs)
* As a poster, I enjoy "contributing" quality content to Lemmy. No one is getting rich or IPOing off my work, and it is clearly designed in a way that even if the founders wanted to make it profitable, they couldn't
Tl;dr For KPIs, Lemmy is objectively worse than Reddit. The only real reason to go is if you like the "alternative" social media services.