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).
Not only pretty markdown editors. Software applications tend to be prettier in general under OSX.
There's the financial incentive, as pointed out in other comments.
Then, what I think it is most important, the technology support. Take just Cocoa, Core Animation and Quartz Composer and you are already light-years ahead of the competition. I won't even talk about things like the Win32 API or the existence of Swift.
'Pretty' Windows apps, in particular, are a royal pain to create. Unless, maybe, if you target Metro.
You can make them pretty even on Windows. However,if you want to keep using native controls, the effort will be enormous. Many applications switched to using embedded browsers because of that. If you are not using standard controls, then you lose things like screen readers.
>'Pretty' Windows apps, in particular, are a royal pain to create. Unless, maybe, if you target Metro.
1. Learn how to work with XAML
(1.5. Know how to make good looking UIs)
2. Done.
Really. If you're forcing yourself to use WinForms, you're doing yourself a disservice. Any Windows Dev that wants to push out good looking application should learn how to use XAML. You can even ignore the databinding part if you want.
That said, TextNut looks an awful lot like OneNote UI wise.
I believe the major problem comes from the fact that most people developing for Windows are either doing it for free or for internal tools. There are very few tools that fit Apple's market of consumer focused applications, and the financial incentive clearly isn't there on Windows.
Perhaps one should try to create the same application on both platforms. I get a feeling that XAML (and WPF) are not a silver bullet and can't make up for deficiencies in other areas. Such as typography.
But my WPF knowledge is limited, so I'll try to keep an open mind and research more when I have the time.
That's been generally how things are on OS X for years (even before the Mac App Store). Small companies like Panic (much smaller originally) have found out that people are willing to pay for quality tools.
Except the Markdown editors phenomenon is slightly different. The tools Panic makes are complex and useful; but there seem to be a huge number of hugely similar "editors" for a format which was designed to be written in plain text. Typically these editors don't add much tangible value over a general purpose text editor and cost twice the price.