Comment by cobertos

Possible factors:

* Composes better than backend does. Frontend software mostly has to "look" right. It can be dropped in. Going the extra mile to integrate is not always necessary (e.g. keyboard navigation, screen reader support, all the stuff that is "easy" to forget).

* Easier to sell in this niche. The overlap between design/creative and software. Designers already have tons of libraries like this. E.g., FontAwesome, Noun Project, font libraries, stock images. They're already looking for new libraries of resources.

* There's also a common language all frontend software has to be written in, and there's a lot of tools for interop if need be. The market is just larger.

Backend software just doesn't compose as well. People expect when they pay for something for it to fit X use case or they leave. Backend has so many more use cases/entry points to get to MVP. It usually works or it doesn't. Perhaps why of the things monetized in the backend (databases, proprietary web servers), most are big monoliths and exist at well-defined software boundaries where things must be serialized to the "wire" to communicate.