Markdown has its own magic incantations that can infuriate if you don't get them just right. GP is right: most people can't handle markup languages, period. Or, they can, but vastly prefer WYSIWYG text formatting.
For me, if I'm writing something for a web page, I use HTML in a decent editor (emacs). Markdown, reST, or anything else is not easier, just different.
If I need a really nice-looking document, I'll probably use LaTeX. But for a simple document or one that I'll work on with other people, I'll normally just use google docs.
If you type plain text, it mostly comes out as plain text.
I've paid John $4.50 for a sandwich.
Renders fine in Markdown. Renders like crap in LaTeX.