Medium wants to serve its subscribers (paying members). So, they always pay attention to that. When too many stories are getting curated, the distribution will be complex and will become a mess. As a result of that, the subscribers will miss out on high-quality, long-term valued stories. The other thing is that the system will not be able to distribute the right posts for the target audience. The system will also receive some false signals to distribute "clickbait" stories that do have the content promised in the title.