Other people are saying "ads," but the story is probably more expansive. Mobile apps have better hooks for pulling people back into the app, like popup notifications. Reddit has natural lifecycle points where they can do this: when people respond to your comments or comment on your posts. They could also notify users of events like their yearly Secret Santa. I wouldn't be surprised if their engagement numbers were much better on mobile: since they can draw users back into the app with notifications, they have better ad views, more people buy Reddit Gold, more people comment (which leads to other people having content to read), etc.