I’m probably wrong but I think because it takes a lot more user effort to navigate Lemmy and find your communities, and those communities can be spread across many instances.
It’s just easier for those that are interested in the community around those interests to use something like reddit or a specific forum site.
Lemmy is mostly tech dorks, which isn’t a bad thing but that leads to the tech and programming communities dominating the feeds. Also I think people who have been using Lemmy for a while vastly overestimate the appeal of the platform and also tech literacy of the general population. It can feel intimidating and uninviting.