While StackOverflow and GitHub users may be tech-savvy, Reddit has a lot of casual users. Markdown is not difficult to pick up, it merely codifies how people type anyway.
Other advantages of using Markdown locally is to separate data from content, legibility, and standard support. Using Pandoc, you can convert Markdown files into HTML, PDF, manpages, etc.
This is a tech community so almost everyone here is likely comfortable with learning new things. The rest of the world isn't like that.
Most people don't like learning new things, particularly if it's not going to really make a difference in their lives in a dramatic way. They are familiar with Word or Gmail.
Think of it this way: let's say bought a new car tomorrow. With this new car, instead just sticking in the gas nossel and filling gas, you had to say a few magic words, stick the nossel in, fill gas and then say a few magic words.
The magic words might be simple and even make sense. Like "adding gas" and "done adding gas". But they are just extra steps. Moving parts. Things that could go wrong. Easily.
I'm saying this primarily because I've used markdown in the past and almost always without fail, I get emails and phone calls about how something isn't working the way it should be and invariably it's one character off from what they intended.