github repo (MIT license): https://github.com/ttscoff/apex
1. Because Apex processes more flavors of Markdown and seems to have more configurability with regard to Markdown syntaxes.
This is not a knock on pandoc. I love pandoc. I use pandoc every single week day.
2. There is no obligation for you to use it. People can create whatever they want and share it with whomever they want.