We may make things more flexible down the line, but for now, it was motivated by two primary reasons:

1. The overwhelming majority of users use one of those three design patterns. For example, I can tell you that more than 98% of GitHub Pages sites use either `master` or `gh-pages` as their primary branch (with only about one tenth of one percent of sites using the `stable` branch).

2. Our experience tells us that every option we add to GitHub Pages increases the learning curve for newer developers. With only those three options, if you're just learning HTML, you don't need to understand Git's branching model before you can create your first website.

We chose these options to start because we thought they struck a good balance between supporting collaborative documentation workflows for open source projects and encouraging "hello world" experimentation among new developers, but as with most features at GitHub, this is just the start.