There is, for me at least, good reason for this. I've long hated having to run a database for the sake of it. I've implemented Wikis a few times on work related placements, and every time I've used something that doesn't need MySQL sat at the back of it. For most situations a few text files is just fine for the purpose.
The fact that I ran Wordpress for a long time was, well, because for the most part it is good. However, I'd thought about rolling something myself (in Perl) for a while.
Then I discovered Octopress. Simple, YAML headered, text files for blog posts, some Ruby wrapping, and layouts all sorted for me. Nice and easy, I like it. Plus, if I want to extend it, it gives me an excuse to do some tinkering with Ruby.
Porting my old Wordpress articles wasn't too difficult either. Google
'wordpress to octopress' and plenty of folks have done it. The only thing that
I did need to sort with the generated markdown files was a lack of a
true setting in the YAML header. I ported all the blog comments to
Disqus, which again was simple (Disqus plugin for Wordpress,
export comments, make sure URLs are the same).
So now it's essentially flat files. Which means I could host it anywhere if I so chose. And writing the posts is nice and simple (MacVim!) and can be tested locally before rsync'ing to the real host.