Mobile users fundamentally engage with the platform in different ways. For one, they form a different demographic segment than older users, and secondly the mobile app frames and filters content in uniquely mobile ways. High-signal content is much more difficult to craft on a mobile device so more mobile use represents a greater proportion of content noise.
Early adopters seek high-signal over content quantity and will move platforms if the opportunity arises. Such a migration will play into future case studies critiquing Reddits MAU-chasing at the expense of hollowing out their long-time user base.