I think this has less to do with Tapastic specifically and more to do with the type of things that generally grab attention. A lot of the most popular featured comics tend to be short, funny, perhaps slice-of-life, and they get a ton of attention because of what everybody else said—they're short, shareable, and more often, relatable. Long-format comics are a greater investment, and it takes a lot more to hook a reader that way. Your characters have to be likable, your story has to be compelling, your art has to grab attention. A "zany" slice-of-life type comic strip can be forgiven on most of those fronts as long as it's funny or relatable. These things apply all over the web, not just tapastic.
The best you can do is just put your comic out there the way it is and don't worry about how many subscribers you get. Your audience will come to you. Another thing to think about is that because long-format comics require more investment, whatever fans you get for your comic will stick around for a long time. Loyal fans are wonderful to have