The daily snack is NOT an algorithm, it is hand picked and occasionally even has a theme.
If you get a daily snack, a staff member will mostly inform you about it via PM beforehand.
A lot of it is luck of being discovered by staff, but both small and big creators have been in the daily snacks so it doesn't appear to be directly related to subs, views, or anything like it. In fact the daily snack is one of the many ways in which Tapas promotes small artists!
The notion that the daily snack only features short form is also false! They have promoted many long forms. However, it may feel like they don't if you post page by page. If you post long form, resort your first few chapters into a few larger updates, and it will raise your chances of ending up in the daily snack. The daily snack promotes and links to specific episodes, and when it comes to long form they often promote the first episode, so that is the episode in question that needs to have enough content, and be enjoyable in a contained piece, so that it will be share-worthy.