For me it's just the most comfortable site. I've been to multiple sites, notably including Webnovel, Scribble, and RoyalRoad, but also other, and Scribble was decidedly the easiest and most comfortable to navigate and write in. It lacks a couple features other sites have, but they aren't any crucial features; I can go about without them just fine.
Webnovel, for example, has its famous contracts and, at least to me, has unintuitive UI and search function. RoyalRoad has weirdly small number of tags, a bit weird algorithm, and has a generally less friendly community.
Really, I think Scribble isn't all that outstanding in any way, but has virtually no cons. Maybe except popularity.