1. Static is secure. Wordpress is a notorious target for hackers and I see constant attacks even against my empty, worthless Wordpress site. But you can't attack a site that doesn't take any input or perform any logic besides choosing which page to display.
2. Markdown is simple and does the job. I don't need anything fancy to write my thoughts and link to resources I find useful.
3. I find Jekyll to be more flexible than Wordpress. Technically both can be used to make any kind of static site, but I find the customization friction to be lower with Jekyll.
4. I like the built-in version control I get with git/Github.
5. I like editing my site with an IDE where all files from content to style are intuitively accessible rather than navigating Wordpress' interface.
6. Github pages includes HTTPS and DDOS protection that I don't need to think about (or even know about). It's just built in.
7. It's in keeping with the open source spirit to have my personal site open source. Anyone can go in and look at what I'm doing, how I'm doing it, and even the history of how the site evolves. They could even fork it if they really wanted to for some reason.