Because they have much fewer subs, and much less accidental activity. So, unless every sub they got is active and perfect, liking and commenting on every chapter without fail, they fall off trending. It’s easier to stay on popular because it is more lenient on the vaunted active engagement.
There is this curve on Tapas. At first, people sub, propelling new book with a few views up. If it doesn’t catch up with silent readers so much that it keeps growing, it falls like a stone. It’s not fun when you hit it.
Alas, it’s only a reflection of the book’s potential. It is a quick turn around, and if you didn’t make it, you are out. Next guy has his chance now.