I think it's a combination of Markdown being completely readable in plain text especially if you're familiar with the syntax and even if you're not. Similarly,

I think it's a combination of Markdown being completely readable in plain text especially if you're familiar with the syntax and even if you're not. Similarly, I see a lot of people using TeX-style mathematics, it's not particularly readable but "The quadratic formula is $x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}$" is a decent way of representing the formula to people fluent in LaTeX even in plaintext conditions. I suppose there's also likely to be a bit of muscle memory where people accustomed to typing in Github/Stack Overflow/Reddit markdown use it on other systems, and even if they see it's not supported it's good enough to not need editing. I don't think it's particularly worthwhile to learn a new comment format (one that's not even linked or described in the comment editor, for that matter) for every site.