As well, I wanted to keep everything in one place. The Local Yarn is (as noted elsewhere) not “a website”; rather it is A) writing, B) print and hypertext designs for the writing, and C) the source code that ties these things together. So I have always wanted to make the code, and the use of the code, a first-class part of the thing that can be explored here. In order to do that properly, I need it to live in the same place as everything else, rather than sending visitors off to an outside host like GitHub or GitLab.
However, you are likely reading this because of the implied second half of the question: why Fossil and not Git?
Being familiar with Git, you may find the following analogy statement instructive:
GitHub hosted repo : Self-hosted Fossil repo :: Expo floor booth : The Batcave
It’s a rough analogy, but it works. Fossil does more than Git, it allows finer control of presentation and security, and is (for better or worse, depending on the project) well outside of the GitHub social network and pull-request system.