Can someone tell me the reason why these people don't want to leave Reddit?

This is actually something I think might be concerning in the long run. Reddit's current direction has driven away a contingent of users who tend to share similar moral values; Lemmy's userbase tends pretty left with a lot of content here being anti-capitalist and pro-marginalized groups. It makes sense that decentralized federated networks would be attractive to those subsets of users.

What I'm afraid of is that this will create a vacuum in which Reddit becomes even more of a breeding grounds for right-wing rhetoric and propaganda without the presence of these users to balance it out a bit. I know that as a Reddit-addicted teen, I hovered dangerously close to some pretty disgusting ideologies. Thankfully I discovered some leftist communities which expanded my narrow worldview and veered me to a much happier path. I don't think reddit as a platform will die, but I fear those communities might, and I shudder to think at what reddit could become without them.