Give them a reason to subscribe. You are asking for a human response here, not an analytic or metadata optimization. Getting someone to subscribe is the result of earning their trust in your content as well as creating a desire to see more. Not only see more; there are plenty of creators our there that I watch all the time that I don't subscribe to because I don't want their content crowding my feeds. When I subscribe I do so because they provide a value that I do not want to miss out on. Recognize that response and create a need for it.
As an aside; I have a lifetime conversion rate of .011 for subscribers, which is a fairly average figure... meaning that you pull 11 subscribers for every 1,000 views.