That doesn't seem to be a source of the issue though. Navigating on to the homepage of the application, the author (of MacDown) writes more about the reason there. Apparently, the author of Mou, wants to stop developing Mou and sell the project and the author of MacDown didn't have the cash and decided to write an open source clone of it instead.
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Why Another Markdown Editor?
I like Mou. I write Markdown all the time, and since I use OS X on a daily basis, Mou is my go-to editor whenever I wish to generate something with markup. But I had always wanted something more.
It came as a great shock when Chen Luo announced that he felt he could not actively continue the development, and wished to sell the ownership of Mou. No suitable offers surfaced (I honestly do not think there will be, either), and I decided that instead of waiting for others to do something about this, I should act myself.
I don’t have nearly enough money to match Chen Luo’s purposed offer, but I do have my own pocket of tricks and some free time. So I started from scratch, spent some weekends hacking together my own solution. And this is the result.
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http://macdown.uranusjr.com/