1. I looked at https://github.com/nelsontky/gh-pages-url-shortener/blob/mai... and it is a basic script that does nothing more than redirection. A lot of URL shorteners on the web do a lot more. Premium ones can track users and route links based on client.
2. Most of URL shortening audience is non-technical and they usually like to pay and forget about it. Hosting and running your own service is an overhead.
3. Handling traffic at scale is challenging. So, if you have a lot of hits, I would not advise spending time maintaining the service.
4. Github can kill this anytime.
That said, this is neat solution for basic needs.