Hm, I think this is a great question, in fact, one that we're asked all the time.
It all starts from an algorithm that aggregates episodes and series that are able to get a certain number of views, subscribes, likes and shares. Each of these are weighted differently and a lot of the times it's in relation to one another (whoa, that's sort of confusing ...). Basically, if your series doesn't get too many views, that's all right so long as it's able to get a healthy ratio of viewer to subscriber, or viewer to likes, or viewers to shares or comments.
Or maybe you've gotten a TON of views but a relatively low amount of engagement (likes, shares, subscribes, comments), then that might actually hurt your chances too. We really want to focus on creators that are able to engage their audience and create a healthy and engaged ecosystem of readers and creators.
Beyond that the staff reads every single series hosted on the site and constantly recommend series to each other. If a lot of the staff agrees that a series is amazing but just isn't getting the traction it deserves, we try our best to help them out - maybe by featuring them on our facebook, or retweeting a link.
Of course, we're always trying to improve how we feature series and more importantly where we feature them.
Hope that helps!