Because I wanted to.
For a long time, I wanted to create myself a blog. I fell in love with the markdown format and started playing around with markdown processors in Python, and creating my blog using Flask. And then I told myself "Hey, it looks pretty easy to transform your blog into a blogging platform". Obviously, I was wrong. I challenged myself, I wanted to have a working blogging platform using the markdown format and a subdomain per user in just three days. It was a success. Since then I just keep pushing new stuff to the platform along with writing articles on my blog.
Of course you can. The code of MarkDownBlog is open-source on GitHub, and is under the MIT license. There is actually
a short tutorial on how to deploy MarkDownBlog on your own server and customizing it a bit in the README.md file.
This was not the primary goal. At all. I'll accept feature requests on GitHub or issues, but keep in mind that this code wasn't originally made to run elsewhere than on my own server. I won't fix issues for you, I won't add features unless I find them fun to add and/or useful. As the code is free, you can modify it to suit your needs.