Maybe they have some little niche communities they do not want to just abandon.
That is why I still use reddit - when you have a really niche hobby or interest and finally discover a community of people that you can share the joy of that interest with, that feels great - you do not want to lose that community that you have longed for for so long.
I am part of some niche communities that have an activity level and a user base that is hard to beat on reddit. I am doing my part here on Lemmy and created local c/dolls and c/denpasong communities on my home instance, but of course nobody is interested here on Lemmy. I am giving Lemmy some time of course, but I am afraid that I will stay lonely with my little communities here on Lemmy unfortunately.
Inside the home even.