Probably because they're easy to make, so there's a lot of traffic there.
I think the real question is "why does the content I see not reflect what I want to see", and I'd guess that that's most-likely because you're browsing "All", which combines all traffic from all communities on any instances that anyone on your home instance, mander.xyz, subscribes to. This is, for any given person, unlikely to be specifically what they are interested in.
I'd generally suggest finding a list of communities that you are interested in, subscribing to them, and then having your webpage/client/whatever set to show "Subscribed" rather than "All".
If you want a convenient way to browse a list of communities on all instances (even ones that nobody on your home instance has subscribed to yet
you can be the first one!), I recommend https://lemmyverse.net/ and clicking on "Communities". If you see one you like, say, !strategy_games@piefed.world, then just search for it on your home instance (like, the text [!strategy_games@piefed.world](/c/strategy_games@piefed.world)), and if your home instance doesn't know about it yet, it'll tell your home instance about it. Then subscribe to it, and traffic there will show up when you browse "Subscribed".
EDIT: Note that Mbin and PieFed communities are, somewhat-unintuitively, shown in their own lists, probably because the three software packages don't provide the same pieces of information about communities and lemmyverse.net wants to let you be able to search using all available search criteria, rather than just the least-common-denominator stuff. If you want to search for communities on Mbin or PieFed, select those from the menu in the upper left.