Part performance and part developer experience.

Gatsby generates HTML pages for each page so it loads as fast or faster than other static site generators but is way faster when clicking around the site as it prefetches pages and renders them in the client so the site feels like a local or native app.

It's also fully setup for building with leading modern web development tools like React, webpack, all CSS options including css-in-js, code splitting, PWA techniques, etc. so the development experience is excellent and you get production quality code without any setup.

Gatsby's ambitions are much higher than simply transforming markdown into HTML. See the writeup of the talk I gave recently describing Gatsby as a website compiler https://www.gatsbyjs.org/blog/2017-09-13-why-is-gatsby-so-fa...