[sooheon]

For one Gruber is arguably the most influential Mac blogger and the creator of Markdown. He and other bloggers like him have promoted and endorsed it. As for the pretty cocoa editors, I'd say the Mac App Store has had a lot to do with it, by allowing developers to get paid for wrapping what used to be a command line workflow into a nice yummy gui. There an entire category of markup powered editors sitting on a spectrum from minimalist "distraction free zen writing environments" (Byword et al) to more featureful writing suites (Ulysses).